Here is something to try, which has worked for me recently:
Go to tty1, login as root, shut down X and make sure no user processes are running.
Then go to the user's home folder and delete ~/.config/kwinrc and ~/.config/plasmarc.
Now use apt to first purge kwin-x11 and plasma-desktop, then to install kwin-x11 and plasma-desktop.
Now try to log in as user. If it works there will be new (default) kwinrc and plasmarc configuration files in your ~/.config directory, so you will have to re-do your systemsettings for the desktop theme, window decorations, etc.