Hello !
Sorry for writing in English, I don't understand German but thanks to google translate I can read your forum. I hope I won't bother you...
2 months ago I installed the v20.0 beta 202209280230 version on my Shield Pro to use Dolby Vision. It worked like a charm but once in a while I lose the media database : I start Kodi and there's only a few show episodes and movies. When I go to the library settings I notice that the network sources for Videos are no longer configured as "This directory contains Movies" or "This directory contains TV shows", but instead they are configured as "This directory contains None". That could explain why almost everything disappears. I then configure them back to what it should be and it takes ages to scan everything again and of course I lose the "watched" status on seen shows/movies.
Is this a bug in that version? I didn't have this problem with v19 from Google Play.
I just installed version 202211050134, I hope the problem will be gone ...