[Updated 2019-11-27] Issues affecting SeriesGuide (reported by you or that I have):
[Resolved 2019-11-15] Only up to 100 episodes of a show are returned.
See the linked post on how to restore watched episodes.
[Partially resolved 2019-11-21] Some shows can not be added because they do not exist (which is incorrect).
It can help to clear the cache for SeriesGuide (Settings > Apps > SeriesGuide > Storage).
[Resolved 2019-11-27] No show poster or poster in wrong language shown.
At least for me and in my tests the highest rated poster in the correct language is now returned for all shows.
[Resolved] Some shows have duplicate seasons (e.g. Money Heist). You may have to remove and add the show.
There continue to be issues with the TheTVDB.com API.
According to their forums API issues are being worked on, which includes images in wrong language/no images at all getting returned.