Comment # 9 on bug 993341 from Dominique Leuenberger

Never mind - I found a way to reproduce the issue quite reliably it seems

in a standard GNOME Session start gnome-tweak-tool and toggle 'Show Application
Menu' in the 'Top Bar' off and back on

The gschema value is changed to: {'Gtk/ShellShowsAppMenu': <1>} (note, the
value for SLESet is lost)

logout and back in (still standard GNOME Session): login fails and you're
thrown back to the gdm

Logging in once using SLE Classic resets the schema 'sufficiently' and be able
to login using normal session again afterwards.

There is clearly mistakes in how we handle non-existing values here.