Hi Götz,
Welcome.
Make sure X-Plane is not running.
You may need to run … your X-Plane 11\X-Plane 11 Installer.exe with Update X-Plane option to check installed files and restore file structure.
In FMS Data Manager, check you are running latest Version 1.8.9.0421 . If not please download and install from FMS Data Manager.
In FMS Data Manager Settings, check your X-Plane 11 path is correct, by navigating to X-Plane.exe.
In Addon Mappings, delete entry X-Plane 11 (11.50+).
Press Scan. Are you now seeing entry X-Plane 11 (11.50+) ?
If yes, in Addon List please select X-Plane 11 (11.50+) and Update.
Cheers
Ian