Black screen on every first GUI session

[jlittle]

... Here results: same problem if for test to create new user then to try start KDE/plasma session for user created newly.

I'd try a clean install, somehow, without overwriting the existing Kubuntu. On btrfs this is very easy, but otherwise you could shrink an existing install, or add a drive internally or externally. This would tell you whether reinstalling is a solution; with problems like this a reinstall often is far quicker than solving the problem.

Indeed I missed this detail. Thank you. Here results: same problem if for test to create new user then to try start KDE/plasma session for user created newly.

Which systemd units to look for in journal log?
KDE / plasma own logging to use instead?

[jlittle]

I suggest adding another user, to see if that user login is affected.

If it's not, you can start comparing settings, and moving settings across.

We are no KDE experts, hence wording we use at some points may be wrong. Sorry for that.

It was 18.04 LTE to 20.04 LTE upgrade upon which KDE/Plasma user session stopped to work.
In general our Kubuntu setup are used in production thus from beginning (we guess it was 16.04) on as few modifications on Kubuntu setup as possible are conducted.

Problem description:
Every first user session following cold boot (from power-off) results in black screen. SDDM works fine - means user can enter their credentials and hit Enter without problems yet it apparently attempts to switch to KDE/plasma session.
What follows is black screen. This is the current problem picture.
Directly after mentioned Kubuntu release picture was yet worse - lot of desktop components were not presented incl. background picture. We had to reinstall plasma-dekstop package or something like this to get current situation.

One recovery measure was found: while in black screen situation got to other TTY console, then back to Plasma session console (e.g. ctrl + alt + F2 followed by ctrl + alt + F1).
Also no problems on every subsequent Kubuntu restart.

As soon as one of recovery measures shown above applied the GUI is rendered fine.

What to use for further troubleshooting?
What to check as next?
Any other proposals, suggestions?
Thank you in advance.