OK, I found something that resolved the problem for me:
I looked through the Plasma settings and found out that "Start compositor on boot" was unchecked for my user in the display settings. Enabling it again and restarting the session resolved the problem completely. Another weird thing was that the file ~/.xsession-errors has a size of about 2.5 GB full of the message "Container window visible, stack below". This is introduced in plasma-workspace 5.23 if I interpret this link right
https://codesearch.debian.net/search?q=Container+window+visible%2C+stack+below&literal=1 Again, thanks for helping me out here. I think the bug can be closed. Best, Patrick
