Ubuntu fails to boot fully, stuck in log screen "ppp link was shut down"

Question

When booting into my Ubuntu 18.04 install after upgrading, I am first faced with a very long boot screen, followed by many messages, the last from GNOME saying something about "the ppp link was shut down".

After switching to tty2 with Alt-F2, I was able to log in but could not enter anything after less than 20 seconds. I was able to regain control by pressing Alt-F7 followed by Alt-F2, but lose it again every 20 seconds.

The journalctl -e showed that gdm3 was restarting every 20 seconds. One of the log entries was ICEAUthFIle exists (or something like it). My system is unusable, how do I fix it?

Solution

I was able to resolve the issue by doing the following things:

a) (probably unrelated, so try only if b does not work):

(dont try before b) sudo apt purge gdm gdm3
sudo apt install gdm3 ubuntu-desktop

b)

1) create a folder /var/lib/gdm3/bak (bak just meaning backup, name is arbitrary)

2) move .ICEauthority .ICEauthority-l and .ICEauthority-c to that folder

sudo mv /var/lib/gdm3/.ICE* /var/lib/gdm3/bak

3) wait a bit (up to a minute to be sure). Might have to press alt + f1 or ctrl + alt + f1 if nothing happens.

Note that I do not know what those files do, they probably contain some configuration (its binary) that would be lost, but the file is recreated by gdm.