Login troubles after upgrade to 16.04

[Guest]

Good evening everyone,

After a successful release upgrade from 15.10, I experience some odd behaviour at logon. This is much slower than it used to be: the login progress bar reaches the end in 1-2 seconds (as is used to do in 15.10), but some extra 20 seconds are needed before the desktop appears. The desktop itself is fully functional.
If I log out and log in again, while kde gets loaded the video stucks on the SDDM login screen and a manual switch to a TTY and back is necessary to refresh and display the desktop correctly. In both cases, the last message in .xsession-errors before the 20s pause takes place is "Failed to retrieve current config: "Failed to prepare backend".

A perhaps related issue concerns the use of "startx" on an empty TTY to start a new X instance. This produces a krunner crash and a plasmashell complain message "all shell packages missing", though this does not happen when starting the session with SDDM.

KDE Plasma 5.5.5
Video card: Intel HD Graphics 5500
Video drivel: i915
Compositing: disabled
Here is my xsession-errors from a SDDM login (edited to show times of messages): http://www.giovanniceribella.eu/0/xsession-errors

Any help is appreciated!

Giovanni

[Guest]

Solved

Update: SOLVED in two passages.
Enabling the compositor and setting its backend either to OpenGL 2.0 or 3.1 (but not XRender) fixed the frozing login screen. The delay in the logon was caused by the plasma desktop applets configuration file in ~/.config/plasma-org.kde.plasma.desktop-appletsrc (I have no clue on why! The desktop itself was configured as a folderview with a few icons in it, and nothing else). It was sufficient to remove the part of that file concerning the desktop applets and the login time recovered to previous values.

On the other hand, invoking "startx" in a TTY still produces a Krunner crash and a plasmashell error. This looks like a separate issue.

Thank you anyhow!
Cheers,

Giovanni