Comment 19 for bug 1803504

I know, it's not well written... Please just try this:

Ctrl+Alt+F4, log in, edit /etc/gdm3/custom.conf and uncomment the line:
  #WaylandEnable=false
Then reboot.

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If the above is too difficult then download all of these:
  https://launchpad.net/ubuntu/+source/mutter/3.28.3-2~ubuntu18.04.2/+build/15618347/+files/gir1.2-mutter-2_3.28.3-2~ubuntu18.04.2_amd64.deb
  https://launchpad.net/ubuntu/+source/mutter/3.28.3-2~ubuntu18.04.2/+build/15618347/+files/libmutter-2-0_3.28.3-2~ubuntu18.04.2_amd64.deb
  https://launchpad.net/ubuntu/+source/mutter/3.28.3-2~ubuntu18.04.2/+build/15618347/+files/mutter-common_3.28.3-2~ubuntu18.04.2_all.deb
  https://launchpad.net/ubuntu/+source/mutter/3.28.3-2~ubuntu18.04.2/+build/15618347/+files/mutter_3.28.3-2~ubuntu18.04.2_amd64.deb

and then run:
  cd ~/Downloads
  sudo dpkg -i *ubuntu18.04.2*.deb