You were correct, the new user does not have these issues.

I did not suspect user settings because I did not change any. Because I did not change any, I don't know which is causing the issues.

Are you aware of any setting being touched by the packages at update? I doubt they touch user settings though... Just what happened? 🤔 I could even wipe them all, if I knew which directories to wipe

Actually, I just remembered that this morning, the older version of plasma was working fine with this home directory. And the first time these issues appeared was when the system updated to the latest plasma packages. Therefore, the new plasma version must not like whatever is contained in these user settings that are perfectly fine with older versions of plasma.

Package updates do not touch user settings directly, but a major KDE or Qt update can make existing configuration files incompatible with the new version — the application then reads an old config it no longer understands and crashes.

The relevant directories are:

~/.cache/ — safe to wipe entirely, it is always regenerated
~/.config/ — KDE and Qt configuration files
~/.local/share/ — KDE local data (including KWallet)

Start with ~/.cache/ alone, as it sometimes resolves issues without losing any settings. If crashes persist, move ~/.config/ and ~/.local/share/ aside (rename, do not delete) and KDE will recreate them from scratch on next login.

Moving .config worked. Unfortunately I have to pick and restore some directories within it. But I guess this case is closed. Thank you for the support