This appears to be a variation of the issue I commented on here → Some Posters not showing up and disappears - #13 by dokuro which so far has been ignored by plex.
Plex at the moment appear to not like it when then top level folder containing a show/movie is renamed, it messes up the artwork.
This may or may not work for you but you can try edit the show/movie in question using the plex ui and select the poster and background tabs and then cancel to close the dialog. Then force refresh the metadata. The artwork should then reload correctly.
If that fails then a plex dance will be needed on the show/movie 
