Black screen after some time with a tiny start menu in upper left corner

When I left my monitor turned off for a while - 10-15 min. and I turn it on, every time I find a black screen with a tiny start menu in upper left corner and no other UI elements.
https://i.imgur.com/risrXn2.png
When I click on Kickoff it actuall works. Alt+F2 is also working.
https://i.imgur.com/DMl14Pw.png
When I change resolution from Kickoff which fortunately is working in this situation to some different than the current resolution, the problem disappears and everything is fine. I can use my PC normally till I turn off my monitor and turn it on after a certain amount of time and then it's starting again - black screen with with a tiny start menu in upper left corner.

This happens with the last updates in a few days. I thought it can be the new Plasma Framework version. I downgraded to 5.73 but this doesn't help. All energy saving options are tuned off. This doesn't help either.

I downgraded to 5.15.0 and the problem is gone.