After a general KDE upgrade, the new systemsettings5 has a black navigation sidebar so it renders unusable. There's actually something written, but since it is black on black, no chance to navigate.
Created attachment 664477 [details] output of emerge --info
Not reproducible, suspected nvidia-drivers issue. Can you try with radeon?
This is after sleep?
Apparently indeed not a Plasma problem.
No, not after sleep, just always. Updating nvidia-drivers to 455.28 fixed the issue.