Thanks!
Now the problematic user is able to save settings for notifications in its user profile…
Strange enough: I gave this user explicit edit-rights although other users obviously do not need this to edit the corresponding user settings. All users are member of a generic group which already gives them all necessary rights…
I still figure this suspicious and may turn this into a fresh user on the next strange behaviour…
Thank you very much again for your patience!
Cheers
Holger