Hm, I totally mixed that up with another issue which was with regular movies.
Let’s confirm something:
Perform the full Plex Dance with one of the videos. (it will lose all metadata, so pick one where it doesn’t hurt much).
If that is successful, we can go on and use the command line to force the scanner to recreate the thumbnails.
https://support.plex.tv/articles/201242707-plex-media-scanner-via-command-line/
--generate --force
