Can you please tell us exactly which version lite / ultimate / barebones and from what date you installed KDE?
With me from the terminal, timeshift opens immediately after asking for the password.
timeshift-launcher
Anwendungskonfiguration geladen: /etc/timeshift.json
/dev/sda2 is mounted at: /run/timeshift/backup, options: rw,relatime,compress=zstd:3,ssd,space_cache,autodefrag,commit=120,subvolid=5,subvol=/
So please use the terminal for this app you want to start and post the output here, maybe we can see what is missing.
edit: librewish know it is a KDE bug, so, maybe it is fixed with next update. 