> Come to think of it, this may be related to the german localization of GDM.
Nope. Setting the language to english
$ sudo LANGUAGE=C gdmsetup
gdmsetup[18534]: CRITICAL: Error opening file: No such file or directory
gdmsetup[18534]: CRITICAL: Error opening file: No such file or directory
gdmsetup[18534]: CRITICAL: Error opening file: No such file or directory
gdmsetup[18534]: CRITICAL: Error opening file: No such file or directory
....
> Come to think of it, this may be related to the german localization of GDM.
Nope. Setting the language to english
$ sudo LANGUAGE=C gdmsetup
gdmsetup[18534]: CRITICAL: Error opening file: No such file or directory
gdmsetup[18534]: CRITICAL: Error opening file: No such file or directory
gdmsetup[18534]: CRITICAL: Error opening file: No such file or directory
gdmsetup[18534]: CRITICAL: Error opening file: No such file or directory
....
shows the same problems.