Login screen goes black after 3 seconds

[glitterball]

I am continuing to experience the same intermittent issue where the SDDM login screen appears briefly and then disappears (black screen) after about 3 seconds.
The mouse pointer remains on the screen.
It doesn't always happen, though this morning I had to hit the reset button 5 times before finally being able to login.

I guess that it is related to my monitors (2 x Acer K212HQL), as I also experienced this on another Kubuntu PC attached to the same monitors.

Any suggestions on how I can debug this? What logs might reveal something?

[rab0171610]

Instead of rebooting to get the login screen back, try going to the tty2:
Hit Ctrl+ALT+F2
Login at the prompt as your user. Then type:
sudo systemctl stop sddm
and then type:
sudo systemctl start sddm
If nothing happens after several seconds,
Hit Ctrl+ALT+F1 to go back to TTY1 where SDDM greeter should be launched. See if the login screen is there.

If nothing is there, go back to tty2 using the instructions from above, and type:
sudo systemctl status sddm
What does it say? You might see ouput similar to this:
- Simple Desktop Display Manager
Loaded: loaded (/usr/lib/systemd/system/sddm.service; enabled; vendor preset: disabled)
Active: active (running) since Mon 2022-05-09 07:29:03 EDT; 44min ago
Docs: man:sddm(1)
man:sddm.conf(5)
Main PID: 16812 (sddm)
Tasks: 4 (limit: 38432)
Memory: 74.5M
CPU: 27.366s
CGroup: /system.slice/sddm.service
├─ 16812 /usr/bin/sddm
└─ 18995 /usr/lib/Xorg -nolisten tcp -background none -seat seat0 vt1 -auth /var/run/sddm/{f3810a04-77ea-4fd1-b>

May 09 07:37:44 rob-B450-AORUS-ELITE sddm-helper[19023]: [PAM] Preparing to converse...
May 09 07:37:44 rob-B450-AORUS-ELITE sddm-helper[19023]: [PAM] Conversation with 1 messages
May 09 07:37:44 rob-B450-AORUS-ELITE sddm-helper[19023]: [PAM] returning.
May 09 07:37:44 rob-B450-AORUS-ELITE sddm[16812]: Authenticated successfully
May 09 07:37:44 rob-B450-AORUS-ELITE sddm-helper[19023]: pam_unix(sddm:session): session opened for user rob(uid=1000) by (u>
May 09 07:37:44 rob-B450-AORUS-ELITE sddm-helper[19023]: pam_env(sddm:session): deprecated reading of user environment enabl>
May 09 07:37:44 rob-B450-AORUS-ELITE sddm-helper[19023]: Starting: "/usr/share/sddm/scripts/Xsession "/usr/bin/startplasma->
May 09 07:37:44 rob-B450-AORUS-ELITE sddm[16812]: Session started
May 09 07:37:44 rob-B450-AORUS-ELITE sddm[16812]: Auth: sddm-helper exited successfully
May 09 07:37:44 rob-B450-AORUS-ELITE sddm[16812]: Greeter stopped.

You can see above the sddm service is active (running) and then the greeter stopped after we logged in. If your output has errors in the lines above or it says not active or dead, please post back with that information.

Also, please post the output of:
sudo journalctl -b | grep sddm

[glitterball]

@rab0171610

Thanks for that - finally feel like I'm getting somewhere with this!
Here's the output of sudo journalctl -b | grep sddm after experiencing the issue again, and following your instructions:

may 09 13:53:29 5700gDesktop sddm[1001]: Initializing...
may 09 13:53:29 5700gDesktop sddm[1001]: Starting...
may 09 13:53:29 5700gDesktop sddm[1001]: Logind interface found
may 09 13:53:29 5700gDesktop sddm[1001]: Adding new display on vt 1 ...
may 09 13:53:29 5700gDesktop sddm[1001]: Loading theme configuration from ""
may 09 13:53:29 5700gDesktop sddm[1001]: Display server starting...
may 09 13:53:29 5700gDesktop sddm[1001]: Adding cookie to "/var/run/sddm/{84f6a177-cebb-40b4-891c-a3ad632a4306}"
may 09 13:53:29 5700gDesktop sddm[1001]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{84f6a177-cebb-40b4-891c-a3ad632a4306} -background none -noreset -displayfd 17 -seat seat0 vt1
may 09 13:53:30 5700gDesktop sddm[1001]: Setting default cursor
may 09 13:53:30 5700gDesktop sddm[1001]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
may 09 13:53:30 5700gDesktop sddm[1001]: Display server started.
may 09 13:53:30 5700gDesktop sddm[1001]: Socket server starting...
may 09 13:53:30 5700gDesktop sddm[1001]: Socket server started.
may 09 13:53:30 5700gDesktop sddm[1001]: Loading theme configuration from "/usr/share/sddm/themes/ubuntu-theme/theme.conf"
may 09 13:53:30 5700gDesktop sddm[1001]: Greeter starting...
may 09 13:53:30 5700gDesktop sddm-helper[1178]: [PAM] Starting...
may 09 13:53:30 5700gDesktop sddm-helper[1178]: [PAM] Authenticating...
may 09 13:53:30 5700gDesktop sddm-helper[1178]: [PAM] returning.
may 09 13:53:30 5700gDesktop sddm-helper[1178]: pam_unix(sddm-greeter:session): session opened for user sddm(uid=119) by (uid=0)
may 09 13:53:30 5700gDesktop systemd-logind[807]: New session 1 of user sddm.
may 09 13:53:30 5700gDesktop systemd[1180]: pam_unix(systemd-user:session): session opened for user sddm(uid=119) by (uid=0)
may 09 13:53:30 5700gDesktop systemd[1]: Started Session 1 of User sddm.
may 09 13:53:30 5700gDesktop sddm[1001]: Greeter session started successfully
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: High-DPI autoscaling not Enabled
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: Reading from "/usr/share/xsessions/plasma.desktop"
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: Loading theme configuration from "/usr/share/sddm/themes/ubuntu-theme/theme.conf"
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: Connected to the daemon.
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: QFont::fromString: Invalid description '(empty)'
may 09 13:53:30 5700gDesktop sddm[1001]: Message received from greeter: Connect
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: Loading file:///usr/share/sddm/themes/ubuntu-theme/Main.qml...
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:53:30 5700gDesktop sddm-greeter[1191]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
may 09 13:53:30 5700gDesktop dbus-daemon[779]: [system] Activating via systemd: service name='org.freedesktop.UPower' unit='upower.service' requested by ':1.34' (uid=119 pid=1191 comm="/usr/bin/sddm-greeter --socket /tmp/sddm-:0-Thxquh" label="unconfined")
may 09 13:53:30 5700gDesktop autojack-start[1290]: Search for: /var/lib/sddm/.config/autojack/autojack.json force: False
may 09 13:53:31 5700gDesktop sddm-greeter[1191]: Adding view for "DisplayPort-0" QRect(0,0 1920x1080)
may 09 13:53:31 5700gDesktop sddm-greeter[1191]: Loading file:///usr/share/sddm/themes/ubuntu-theme/Main.qml...
may 09 13:53:31 5700gDesktop sddm-greeter[1191]: QQmlEngine::setContextForObject(): Object already has a QQmlContext
may 09 13:53:31 5700gDesktop sddm-greeter[1191]: QQmlEngine::setContextForObject(): Object already has a QQmlContext
may 09 13:53:31 5700gDesktop sddm-greeter[1191]: Adding view for "DisplayPort-1" QRect(1920,0 1920x1080)
may 09 13:53:31 5700gDesktop sddm-greeter[1191]: Message received from daemon: Capabilities
may 09 13:53:31 5700gDesktop sddm-greeter[1191]: Message received from daemon: HostName
may 09 13:53:34 5700gDesktop sddm-helper[1178]: [PAM] Closing session
may 09 13:53:34 5700gDesktop sddm-helper[1178]: pam_unix(sddm-greeter:session): session closed for user sddm
may 09 13:53:34 5700gDesktop sddm-helper[1178]: [PAM] Ended.
may 09 13:53:34 5700gDesktop sddm[1001]: Auth: sddm-helper exited with 9
may 09 13:53:34 5700gDesktop sddm[1001]: Greeter stopped.
may 09 13:55:01 5700gDesktop sudo[1579]: MyUsername : TTY=tty2 ; PWD=/home/MyUsername ; USER=root ; COMMAND=/usr/bin/systemctl stop sddm
may 09 13:55:01 5700gDesktop sddm[1001]: Signal received: SIGTERM
may 09 13:55:01 5700gDesktop sddm[1001]: Socket server stopping...
may 09 13:55:01 5700gDesktop sddm[1001]: Socket server stopped.
may 09 13:55:01 5700gDesktop sddm[1001]: Display server stopping...
may 09 13:55:01 5700gDesktop sddm[1001]: Display server stopped.
may 09 13:55:01 5700gDesktop sddm[1001]: Running display stop script "/usr/share/sddm/scripts/Xstop"
may 09 13:55:01 5700gDesktop systemd[1]: sddm.service: Deactivated successfully.
may 09 13:55:13 5700gDesktop sudo[1584]: MyUsername : TTY=tty2 ; PWD=/home/MyUsername ; USER=root ; COMMAND=/usr/bin/systemctl start sddm
may 09 13:55:13 5700gDesktop sddm[1607]: Initializing...
may 09 13:55:13 5700gDesktop sddm[1607]: Starting...
may 09 13:55:13 5700gDesktop sddm[1607]: Logind interface found
may 09 13:55:13 5700gDesktop sddm[1607]: Adding new display on vt 1 ...
may 09 13:55:13 5700gDesktop sddm[1607]: Loading theme configuration from ""
may 09 13:55:13 5700gDesktop sddm[1607]: Display server starting...
may 09 13:55:13 5700gDesktop sddm[1607]: Adding cookie to "/var/run/sddm/{cbeec067-2379-4052-99c8-427da0229b5e}"
may 09 13:55:13 5700gDesktop sddm[1607]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{cbeec067-2379-4052-99c8-427da0229b5e} -background none -noreset -displayfd 17 -seat seat0 vt1
may 09 13:55:14 5700gDesktop sddm[1607]: Setting default cursor
may 09 13:55:14 5700gDesktop sddm[1607]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
may 09 13:55:14 5700gDesktop sddm[1607]: Display server started.
may 09 13:55:14 5700gDesktop sddm[1607]: Socket server starting...
may 09 13:55:14 5700gDesktop sddm[1607]: Socket server started.
may 09 13:55:14 5700gDesktop sddm[1607]: Loading theme configuration from "/usr/share/sddm/themes/ubuntu-theme/theme.conf"
may 09 13:55:14 5700gDesktop sddm[1607]: Greeter starting...
may 09 13:55:14 5700gDesktop sddm-helper[1639]: [PAM] Starting...
may 09 13:55:14 5700gDesktop sddm-helper[1639]: [PAM] Authenticating...
may 09 13:55:14 5700gDesktop sddm-helper[1639]: [PAM] returning.
may 09 13:55:14 5700gDesktop sddm-helper[1639]: pam_unix(sddm-greeter:session): session opened for user sddm(uid=119) by (uid=0)
may 09 13:55:14 5700gDesktop systemd-logind[807]: New session 5 of user sddm.
may 09 13:55:14 5700gDesktop systemd[1641]: pam_unix(systemd-user:session): session opened for user sddm(uid=119) by (uid=0)
may 09 13:55:14 5700gDesktop systemd[1]: Started Session 5 of User sddm.
may 09 13:55:14 5700gDesktop sddm[1607]: Greeter session started successfully
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: High-DPI autoscaling not Enabled
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Reading from "/usr/share/xsessions/plasma.desktop"
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Loading theme configuration from "/usr/share/sddm/themes/ubuntu-theme/theme.conf"
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Connected to the daemon.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QFont::fromString: Invalid description '(empty)'
may 09 13:55:14 5700gDesktop sddm[1607]: Message received from greeter: Connect
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Loading file:///usr/share/sddm/themes/ubuntu-theme/Main.qml...
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Adding view for "DisplayPort-0" QRect(0,0 1920x1080)
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Loading file:///usr/share/sddm/themes/ubuntu-theme/Main.qml...
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QQmlEngine::setContextForObject(): Object already has a QQmlContext
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QQmlEngine::setContextForObject(): Object already has a QQmlContext
may 09 13:55:14 5700gDesktop autojack-start[1753]: Search for: /var/lib/sddm/.config/autojack/autojack.json force: False
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Adding view for "DisplayPort-1" QRect(1920,0 1920x1080)
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Message received from daemon: Capabilities
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Message received from daemon: HostName
may 09 13:55:41 5700gDesktop sddm-greeter[1652]: Reading from "/usr/share/xsessions/plasma.desktop"
may 09 13:55:41 5700gDesktop sddm[1607]: Message received from greeter: Login
may 09 13:55:41 5700gDesktop sddm[1607]: Reading from "/usr/share/xsessions/plasma.desktop"
may 09 13:55:41 5700gDesktop sddm[1607]: Reading from "/usr/share/xsessions/plasma.desktop"
may 09 13:55:41 5700gDesktop sddm[1607]: Session "/usr/share/xsessions/plasma.desktop" selected, command: "/usr/bin/startplasma-x11"
may 09 13:55:41 5700gDesktop sddm-helper[1798]: [PAM] Starting...
may 09 13:55:41 5700gDesktop sddm-helper[1798]: [PAM] Authenticating...
may 09 13:55:41 5700gDesktop sddm-helper[1798]: [PAM] Preparing to converse...
may 09 13:55:41 5700gDesktop sddm-helper[1798]: [PAM] Conversation with 1 messages
may 09 13:55:41 5700gDesktop sddm-helper[1798]: pam_kwallet5(sddm:auth): pam_kwallet5: pam_sm_authenticate
may 09 13:55:41 5700gDesktop sddm-helper[1798]: [PAM] returning.
may 09 13:55:41 5700gDesktop sddm[1607]: Authenticated successfully
may 09 13:55:41 5700gDesktop sddm-greeter[1652]: Message received from daemon: LoginSucceeded
may 09 13:55:41 5700gDesktop sddm-helper[1798]: pam_kwallet5(sddm:setcred): pam_kwallet5: pam_sm_setcred
may 09 13:55:41 5700gDesktop sddm-helper[1798]: pam_unix(sddm:session): session opened for user MyUsername(uid=1000) by (uid=0)
may 09 13:55:41 5700gDesktop sddm-helper[1798]: pam_kwallet5(sddm:session): pam_kwallet5: pam_sm_open_session
may 09 13:55:41 5700gDesktop sddm-helper[1801]: pam_kwallet5: final socket path: /run/user/1000/kwallet5.socket
may 09 13:55:41 5700gDesktop sddm-helper[1798]: Starting: "/etc/sddm/Xsession "/usr/bin/startplasma-x11""
may 09 13:55:41 5700gDesktop sddm-helper[1803]: Adding cookie to "/home/MyUsername/.Xauthority"
may 09 13:55:41 5700gDesktop sddm[1607]: Session started
may 09 13:55:41 5700gDesktop sddm-helper[1639]: [PAM] Closing session
may 09 13:55:41 5700gDesktop sddm-helper[1639]: pam_unix(sddm-greeter:session): session closed for user sddm
may 09 13:55:41 5700gDesktop sddm-helper[1639]: [PAM] Ended.
may 09 13:55:41 5700gDesktop sddm[1607]: Auth: sddm-helper exited successfully
may 09 13:55:41 5700gDesktop sddm[1607]: Greeter stopped.

[rab0171610]

Let me know if switching to tty2 and stopping and starting sddm service allows you to get the greeter back.

Because the only thing at first glance I see of interest:

may 09 13:55:14 5700gDesktop sddm[1607]: Message received from greeter: Connect
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Loading file:///usr/share/sddm/themes/ubuntu-theme/Main.qml...
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QObject: Cannot create children for a parent that is in a different thread.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
may 09 13:55:14 5700gDesktop sddm-greeter[1652]: Adding view for "DisplayPort-0" QRect(0,0 1920x1080)

You could also try deleting the sddm qml cache dir:

sudo rm -fr /var/lib/sddm/.cache/sddm-greeter/qmlcache

See if that fixes the problem?

[rab0171610]

If you are not sure about deleting it, you can also move it temporarily:
sudo mv /var/lib/sddm/.cache/sddm-greeter/qmlcache /var/lib/sddm/.cache/sddm-greeter/qmlcache.bak
I should have warned you that anytime you use sudo + fr or sudo + fr you need to be very careful!!!

[glitterball]

Let me know if switching to tty2 and stopping and starting sddm service allows you to get the greeter back.

It did indeed get the greeter back - and I didn't need to switch back to tty1.

I'll try deleting the cache and see if the problem comes back (it is intermittent - It will happen every day, and then go several days without happening).

[glitterball]

I deleted that cache, and immediately experienced the issue again. After rebooting for a second time, the login screen appeared correctly.

[rab0171610]

Fingers crossed.

[glitterball]

Still occurring unfortunately.

Looking through that log, the part where the screen goes black is this:
may 09 13:53:34 5700gDesktop sddm-helper[1178]: [PAM] Closing session
may 09 13:53:34 5700gDesktop sddm-helper[1178]: pam_unix(sddm-greeter:session): session closed for user sddm
may 09 13:53:34 5700gDesktop sddm-helper[1178]: [PAM] Ended.
may 09 13:53:34 5700gDesktop sddm[1001]: Auth: sddm-helper exited with 9
may 09 13:53:34 5700gDesktop sddm[1001]: Greeter stopped.

It seems to be totally unprovoked!

Seems to be a known bug:
https://bugs.launchpad.net/ubuntu/+s...m/+bug/1893302

[rab0171610]

Some things you could try:
switch to tty2
stop the sddm service:
sudo sytemctl stop sddm
delete the .Xauthority file in your home folder and reboot:
sudo reboot
and see if that helps
The file should recreate itself when Sddm tries to start on reboot.
You can also switch to tty 2
stop the sddm service
list all the sessions in use and then use second command replacing id with session numbers, try them all:

sudo -i loginctl list-sessions

sudo -i loginctl unlock-session id

The first command will list sessions like so, use the session id's with the second command above replacing the last word, id, with all of the session numbers:
SESSION UID USER SEAT TTY
10 1000 rab seat0
2 1000 rab seat0
6 1000 rab seat0 tty2
8 1000 rab seat0

so 10, 2, 6, 8
for example:
sudo -i loginctl unlock-session 10
(or 2, or 6 or 8 from the list above)

or you could just unlock all sessions:
sudo loginctl unlock-sessions
and see if any of that helps.
I am just grasping at straws here.
These ideas to try came from the Arch sddm wiki:

Troubleshooting

Blank screen with cursor, but no greeter shows


Greeter crashes if there is no available disk space. Check your disk space with df -h.

If disk space is not the issue, it may be due to a bug. Switch to another TTY, and then try loginctl unlock-session session_id or to restart SDDM.

We have already gone through restarting and stopping SDDM
Probably won't help, but hey, who knows until they try?

[rab0171610]

There is also a way to test themes for sddm from the command line which may be able to see if it is one particular theme that might be corrupt. You could always search for how to do this in google. This would eliminate the possibility it is a theme issue.

[glitterball]

I probably should have been more clear about the fact that I have been experiencing this issue for over a year on two separate machines - through various clean installs of the last 3 versions of Kubuntu.
The common denominator is the 2 x Acer K212HQL monitor setup (everything else is different). I don't experience it on any of the laptops when they are not connected to this monitor setup.

[oshunluvr]

What about turning off one screen in X during boot up so SDDM only "sees" one screen? Obviously, if you're using Wayland, this won't have any effect.

***Disclaimer*** I have not tried this since I'm running a single monitor at this time. Also, this info is a couple years old, so there might be a "newer" way to do it. However, I don't believe this will break anything so you would be able to undo it if it does not have the desired effect.

First, determine which monitor you DON'T want the greeter on in X. This command will list the inputs:

xrandr -q

The output will look something like this (depending on how many ports you have):

Screen 0: minimum 320 x 200, current 3840 x 1080, maximum 16384 x 16384
DisplayPort-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-1 disconnected (normal left inverted right x axis y axis)
DisplayPort-2 disconnected (normal left inverted right x axis y axis)
HDMI-A-0 disconnected (normal left inverted right x axis y axis)
DVI-D-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-3 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 880mm x 367mm
1920x1080 60.00*+ 50.00 59.94
1680x1050 59.95
1280x1024 75.02 60.02
1440x900 59.89
1280x960 60.00
1280x800 59.81
1152x864 75.00
1280x720 60.00 50.00 59.94
1024x768 75.03 70.07 60.00
832x624 74.55
800x600 72.19 75.00 60.32 56.25
720x576 50.00
720x480 60.00 59.94
640x480 75.00 72.81 66.67 60.00 59.94
720x400 70.08
DisplayPort-4 connected secondary 1920x1080+1920+0 (normal left inverted right x axis y axis) 880mm x 367mm
1920x1080 60.00*+ 50.00 59.94
1680x1050 59.95
1280x1024 75.02 60.02
1440x900 59.89
1280x960 60.00
1280x800 59.81
1152x864 75.00
1280x720 60.00 50.00 59.94
1024x768 75.03 70.07 60.00
832x624 74.55
800x600 72.19 75.00 60.32 56.25
720x576 50.00
720x480 60.00 59.94
640x480 75.00 72.81 66.67 60.00 59.94
720x400 70.08

Then, edit /etc/sddm.conf and add this:

[XDisplay]
DisplayCommand=/usr/share/sddm/scripts/Xsetup

Then edit /usr/share/sddm/scripts/Xsetup and add this:

xrandr --output PORT --off

where "PORT" is your screen to turn off. Using the example output above, it would be:

xrandr --output DisplayPort-4 --off

The end result should be that SDDM will only use monitor 1 and monitor two will be off. Once you log in, the second monitor should come on normally.

[rab0171610]

I probably should have been more clear about the fact that I have been experiencing this issue for over a year on two separate machines - through various clean installs of the last 3 versions of Kubuntu.
The common denominator is the 2 x Acer K212HQL monitor setup (everything else is different). I don't experience it on any of the laptops when they are not connected to this monitor setup.

Indeed. That means nothing I suggested will have any effect on your situation. However, oshunluvr's advice looks very promising in this setup.

[glitterball]

@oshunluvr
I tried your suggestion of editing /etc/sddm.conf and /usr/share/sddm/scripts/Xsetup , but unfortunately I am still experiencing the issue (tried both monitors).