Pity that did not work 
Did you try the script from @SwiftPanda16 → Server Version 1.40.1.8111 has broken poster/artwork & thumbnails - #26 by SwiftPanda16
There has been a looooooooooooong running problem with plex and posters which I outlined in this post Some Posters not showing up and disappears - #13 by dokuro where posters become blank. Pure speculation on my part but I suspect this issue has exposed that problem and when it happens its hard to recover. As I use local assets its somewhat easier. Maybe the script from SwiftPanda will help you.
