I just did a complete rescan of the library after removing old sources and adding new ones, and many of the movies and TV shows have blank posters. I have also encountered this problem with individual movies/TV shows in the past even without doing a global update/rescan.
This could be a caching issue, because the problem is solved after going to Refresh art, choose poster, and pressing Remote poster (even though remote and local are identical).
If this has already been reported, please merge into an existing topic.