org.gnome.desktop.screensaver.gschema.xml.in has outdated picture-uri
The key 'picture-uri' is still set to 'file://@datadir@/backgrounds/gnome/adwaita-lock.jpg' which is outdated and doesn't exist anymore in current gnome-backgrounds-3.28+. Should be 'file://@datadir@/backgrounds/gnome/adwaita-timed.xml' now.