SUMMARY Explanation here: https://unix.stackexchange.com/q/666617/233262 I already solved this by removing (or moving) the /home/username/.local/share/plasma/plasmoids/org.kde.plasma.eventcalendar/ folder (e.g. by running dolphin ~/.local/share/plasma/plasmoids/). Would be good to check if plasmoids / eventcalendar can cause issues. STEPS TO REPRODUCE 1. Upgrade from Debian 10 to 11 (not sure if older versions of some packages are required to reproduce it, it must have occurred at the time that I posted this question linked above) 2. Boot with KDE OBSERVED RESULT Black screen after a while EXPECTED RESULT No issues at all after upgrading SOFTWARE/OS VERSIONS Linux/KDE Plasma: Debian 11 KDE Plasma Version: KDE Frameworks Version: Qt Version: ADDITIONAL INFORMATION I know it's old software and it may already be fixed. This could be helpful if you can easily find an issue with the eventcalendar for example and quickly know what caused it.
> Black screen after a while So it happens after you log in, and after you've been able to use the system for a while? Regardless, I'm afraid Plasma 5.20.5 is no longer maintained because it's almost impossible to debug issues like this on old versions of Plasma. Debian has assumed the burden of support for the old software they ship, so please file bugs for them, or else try again with Debian Testing.