The TVDB guys are rude and won’t change it:
https://forums.thetvdb.com/viewtopic.php?p=41889#p41889
So I changed the agent to TMDB (after removing the complete tv shows as changing the agent does not affect existing content, why?) and now its correct. Thanks!
Regarding your suggestion to rename all files:
1.) Of course I could rename the season-folders. But not everyone in my household understands English and as long it works I use the German translation.
2.) But adding the tv show name and year additionally to all episode file names does not make sense to me in a semantic point of view. In German I would say “doppelt gemoppelt” (~exaggerated organized). I really don’t hope Plex will ever force users to name their files like that ^^
