gnome – gdm3 not starting on ubuntu 20.04

I recently upgraded from Ubuntu 18.04 to 20.04 as I was having this issue as well (with a different error). I was hoping upgrading would fix it. After upgrading Ubuntu worked a few times but now it won’t start. I’ve been trying for ages to fix this including un-commenting #WaylandEnable=false, however when I tried that the laptop wouldn’t start at all and stayed on the line dev/sda1: clean, xxxxxx/xxxxx files, xxxxxxx/xxxxxxx blocks and wouldn’t go any further.

It might help to know this is a very old laptop >10years dell xps. Ubuntu works fine when it starts and is the only OS installed.

The output of systemctl status gdm is (I am typing this so might be errors):

Started GNOME Display Manager.
gdm-launch-environment (963): pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
gdm3(958): Gdm Display: Session never registered, failing
gdm3(958): Child process -1011 was already dead.
gdm3(958): Child process -1011 was already dead.
gdm-launch-environment (1324): pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
gdm3(958): Child process -1346 was already dead.

I would really like to get this fixed as it is a laptop I use a lot. I also really quickly tried to install lightdm but ubuntu told me there isn’t enough disk space in /var/cache/apt/archives (which I am pretty sure is untrue last I checked I had about 9GB free) but I can delete some stuff and install lightdm if that is the best fix.

xorg – Restarting the GUI (GDM3) on Ubuntu via remote SSH

Given an Ubuntu 18.04.2LTS on a desktop computer (not headless) which haven’t been restarted since at least months.

Access is only possible via some remote desktop connection (like vnc) and SSH.

Now the rdp connection was faulty, and I decided to (not so wisely) restarting the gdm3:

service gdm3 restart

But after this command, rdp is not working at all, it seems the GUI is not running.

I have no physical access to the computer, and rebooting is also not possible remotely.

How can I properly (re)start Ubuntu 18.04’s default GUI from remote SSH without restarting the computer?

It would be even better to restart the GUI “at the login prompt”: force logging out all users, and start as fresh as a clean startup would do.

Please note I have already activated Xorg instead of Wayland long time ago by this:

# Uncoment the line below to force the login screen to use Xorg

How can I fix the GDM3 loop?

After following the advice here, I'm caught in a loop. Only the Wayland version works, but I need the normal edition. I no longer care about getting KDE, I just want my laptop to be fixed. It has an old 2013 Nvidia Quadro if that helps.