I've got the same problem here. The problem is switching to GUI mode / X-Windows greeter. I'm not sure what change there was coming from 18.04 LTS to 20.04 LTS.
Ctrl + Alt + F2 brings up the console and the system is running.
- Removing guest tools didn't help
- Installing guest tools didn't help
So I then turn off the splash screen (Plymouth) so I can see what's happening at boot
Code:
sudo -i
vim /etc/default/grub
remove 'quiet splash' from GRUB_CMDLINE_LINUX_DEFAULT="". Leave the quotes
update-grub
Now I can see that the boot gets to "[ OK ] Reached Target Graphical Interface." and doesn't do much after that.
Now on to figure out how to get X-Windows runnning...