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?