The problem is with 18.10 cosmic desktop after a do-release-upgrade. Before the release upgrade I did not have this issue, or at least I did not notice it.
The issue is that when the OS starts, the graphical interface just won't start no matter how long I wait. I only see an empty black screen with a blinking cursor at the top left corner of the screen. GUI definitely worked before the release upgrade, althoug I had occassions where I needed to manually switch to the graphical terminal. This happened rarely, only a handful of times, so I didn't bother to hunt the problem down.
I found a strange workaround: pressing CTRL+ALT+F2 and then CTRL+ALT+F1 switches terminals, and shortly the GUI starts up properly.
Then I accidentally noticed that there are not one, but two GUIs are running, one on VT #1 (accessible with CTRL+ALT+1), and an other one on VT #3 (CTRL+ALT+3).
If I run the command
ps fauxww I can clearly see two xorg instances running on vt1 and vt3, forked by gdm-x-session worker processes, those were started by their respective gdm-session-worker processes, both of which are started by a single gdm3 process.
Is this normal? (Not likely...) I think there should only be a single GUI process. Could the multiple running GUI instances have anything to do with not being able to start up properly?