The joy of running #debian #unstable: upgraded after half a year and had to accept a few removals during the upgrade but nothing major... Restarted, no login screen anymore. Turned out GDM/Gnome shell needs weather location data and that package was not installed when installing gdm/gnome-core... And google shows only "autologin" and "dconf corrupted" related issues :-(
(And of course, when you finally know what the problem is, there is already a bugreport for it: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1151506 :-))