Update 2019-08-05: all issues should be resolved. It may take a little bit for your shows to update.
On Monday, 2019-07-29 the folks at TheTVDB.com have gone live with a new version of their site (and apparently API). As most of you have noticed (and now me, too, sorry, I was on holiday) many things did go wrong and they decided to roll back to the old version.
There are currently still issues, e.g. posters not displaying (the TheTVDB API currently returns an empty file name).
For now we can only wait until the TheTVDB folks get things sorted out.