Um, I was a bit lost when I last read your question, so I didn’t chime in when you first posted. But it seems like nobody else has any ideas either, so let me ask a question of two to clarify.
Did the thumbnails generate at all? You say “everything works fine” on the plex app for Android phone, but that seems to me to be an odd thing to say when you are trying to see the video thumbnails.
One thing to try is to Plex Dance one show/movie in particular. Keep an eye on the Activity icon in the upper right of a browser when you re-add the file/files. Make sure that you see the “Generating video thumbnails” action happens. For me on my beefy server, it occurs pretty quick per file (5 seconds per half-hour episode), so keep a close eye on it while you add the files back in.
