The new Plex TV Series Agent does not download season posters from thetvdb.com (unlike the old TheTVDB agent). For example the show Die schlechtesten Filme aller Zeiten - TheTVDB.com hat a seasonal poster for seasons 1-8, but they are not downloaded by the new agent. The old one did so without problems.
As I’ve seen so far all season posters of all shows have disappeared!
But it doesn’t return anything. If I use a different show, like Game of Thrones, then I get back season posters.
I quickly checked and this doesn’t work anymore on the old TVDB agent either, you must have had those posters downloaded before the api broke/changed.
I could open a ticket with TVDB but I doubt they’re going to fix this as they’re trying to get their v4 api out the door, we’ll be using the new API also once it’s ready.
UPDATE:
After a bit more tinkering I can get the poster to return by adding the Accept-Language: de header as I see the seasons are only available in German. We might be able to fix this on our side then, I’ll find out.