Ubuntu 15.10 and broken Gnome session

No Gnome session coming up at all after a recent upgrade to Ubuntu 15.10 on workstation with AMD 6850HD (using radeon drivers): GDM3 would not take over any VT at all, the text output from systemd startup would just freeze, no VT switching possible, even though the system itself was alive and well – reacheable via SSH.

Solved by switching to lightdm as default display manager:

dpkg-reconfigure lightdm

and choosing lightdm over GDM3 as default.

Go figure.