1.18.2.2058 has been released today and is supposed to provide a first hotfix while TheTVDb.com is working to fix their API which was broken when they tried to update it. From what I’ve seen, matching and basic metadata download should work but posters/backdrops/thumbs might still cause trouble (those should download fine later when you refresh metadata – occasionally already working; occasionally depending on the pending fix of their API 
).
From the Plex Media Server - #303 by StSimm1 release announcement:
FIXES:
- (Agents) Make the Framework use modern HTTP libs, fixes certain TheTVDB requests failing among others.
