Interesting, I have encountered an "Failed to start session" error.
Right now, I am using the same laptop, only the basic 'ubuntu-desktop' on Wayland session. Regular Ubuntu option - the one under XORG, I guess - met me with the display crash I have already mentioned somewhere around on this forum.
This way or the other, I was messing around in the Boutique, installed the (GNOME) Software assistant, removed 'Orca' and stuff, then I have uninstalled the (GNOME) assistant through the Boutique. Afterwards, I tried to install 'Flightgear', but the installation got stuck somewhere, so I cancelled it. Always do I run 'apt autoremove' and 'apt autoclean' after uninstalling anything.
This way or the other, I have gone the sleep mode, returned sometime later, to conclude that the session would not begin. So I switched to terminal >CTRL+ALT+F1 and having logged in, executed "reboot". Then the problem would appear.
I got some intel on another computer, tried to run couple of different commands, eventually just installed 'ubuntu-desktop' and this is how the story loops.
Any idea how can I get back to MATE desktop?
EDIT:
Y'know what, the Ubuntu desktop is pretty ■■■■, good job! Do not know why I did not like it like back in the day. Maybe I just did not want to. But I would stick to MATE anyway already, the super-duper colors on default can get one weary. But the Wayland does seem to be somewhat more tight visually.