Hi there !

I don't know why, but a KDE session fail to start properly in my Ubuntu 18.04.
I thought it was a problem with one of my .kde configuration files and them, the first thing I've tried is to create a new "tst" user with a fresh home area and using this user, to login in, but the problem remains, same behavior: the desktop background is showed for a moment, no icons, and them it changed to a black screen. I've to CTRL-ALT-DEL to bring the Plasma logout screen, which is working!!! Two shots of the screen if provided if it is any help.

Them I've tried to change the session from Plasma to ubuntu and it worked, both for my regular user as well to the "tst" user.

On the Ubuntu session I've tried to change the video driver from proprietary nvidia-driver-390 to nouveau xorg driver but no change at all. After a reboot and them a plasma login session the screen shows up the default background, no icons and them a black screen.

Back to the nvidia-driver-390, I boot on single mode, forced fsck with "c" flag on boot, usr and home partitions. No good.

I've tried to uninstall kde and re-install it, maybe some corrupted file ? (BTW, I have not experienced any freezes, power outages, crashes or anything like that than can explain this)

The commands I used was:

No good. Same shit !

Next, I tried to install the kubuntu backports to test a new version of kde plasma ("deb http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu bionic main") followed the same commands above to uninstall the previous version and re-install the new one from kubuntu backport ppa. No way. The same thing: the default background with no icons and after 2 seconds, a black screen with the mouse pointer. Again, CTRL-ALT-DEL is working as expected and it brings the KDE logout screen. After the logout, the default kde's login screen is showed again (sddm).
Using the user tst still is no good, even after clearing the configuration files on its home (.config, .kde, etc)

My last hope is someone here that have experience with debugging this sort of problem can analyze the xsession-errors files attached to this post (from the "clean" user "tst" ) and found the problem. There is what looks like error messages on it, but I am not sure if they are errors messages indeed or only normal warnings....and, if errors, how to fix.

I appreciate any help.

PS: Looks like there is a problem with attached files here.
Please try this one: background: https://file.io/tObyJY
back screen: https://file.io/YodRm1
nouveau default ubuntu: https://file.io/Tg3uxI
kubuntu ppa: https://file.io/0AvZ6e

Have you looked in the log files? I'd start with /var/log/Xorg.0.log and /var/log/syslog.

I'd recommend the tail command as a useful way to start.