Found the error.
Just in case anybody else ever stumbles over that problem: It was an environment variable that I myself put into place because of obs and wayland tests in /etc/environment
→ QT_QPA_PLATFORM=xcb obs
my sddm and kde did work without problem, even through reboots. Somehow, something in the update broke, but without this, it works again!
So, I ended up removing obs and yeah, thats that.