Apparently when you select a custom wallpaper the Background value in AccountsService gets updated to an entry below $HOME/.cache/gnome-control-center/backgrounds/ that is not world-readable.
In contrast, if you select a standard wallpaper, the Background value gets set to a (world-readable) entry below /usr/share/backgrounds/
I can reproduce that.
Apparently when you select a custom wallpaper the Background value in AccountsService gets updated to an entry below $HOME/. cache/gnome- control- center/ backgrounds/ that is not world-readable.
In contrast, if you select a standard wallpaper, the Background value gets set to a (world-readable) entry below /usr/share/ backgrounds/