Ive got the same problem, and was able to fix it.
/var/lib/plasmalogin/wallpapers/
The owner of this folder for me was root, it should be plasmalogin manager,
No idea how or why it happened but after changing the owner of the folder it worked again.
Gave full permissions to anyone as a test and it woirke, after checking the folder again it had 755 and plasmalogin as the user, so simply changing the user to plasmalogin should fix it.