I have just upgrade 18.04 to 20.04, which went fine. But when I tried to log in, it would just dump me back at the login screen again. Looking through the logs I saw the line
(EE) xf86OpenConsole: Cannot open virtual console 7 (Permission denied)
The permissions for /dev/tty7 were fine. Various pages suggested solutions like removing .Xauthority, editing /etc/X11/Xwrapper.config, re-configuring gdm3 or uninstalling and reinstalling gdm3 and ubuntu-desktop-simple. This place has got loads that didn’t work for me.
Anyway, long story short – I noticed that there was a big delay logging in to a console. Sometimes this can be because it’s trying to mount a remote filesystem with NFS and timing out. I had 4 entries – two of which were for a computer which was turned off. They both had the “noauto” flag set, meaning they shouldn’t be mounted automatically but they were still messing up the login procedure. Commenting them out fixed the problem and I could log in normally.