The #ubuntu login loop that I encountered about a month ago is caused by specifying dependencies in a systemd automount file.
The visible symptom is a GDM login loop.
Another symptom is that /tmp/.X11-unix and /tmp/.ICE-unix will be owned gdm/gdm, instead of root/root. Changing the ownership to root/root fixes the login loop, but it returns after the next reboot.
All the gory details are spelled out here.