Currently there is a bug in 20.04 that causes the desktop xorg session to crash and drop the user back to the GUI Login screen.
This bug occurs with several programs and is not limited to Ubuntu MATE. However, it is particularly troublesome for us as Welcome is one of the programs that this issue affects. It is possible for a user running Ubuntu MATE 20.04 in a Virtual Machine to come upon a login-crash loop.
While this is being worked out, there are a couple of workarounds.
On the first boot after installation, before logging into the machine, drop into a TTY session login and remove the
gstreamer-vaapiapplication. Then the user can login to the machine and not have the possibility of this particular login crash loop.
At the GUI Login screen:
Ctrl + Alt + F1 to enter a TTY.
Login with your username and password and do the following:
sudo apt-get purge gstreamer1.0-vaapi sudo apt-get autoremove --purge
Ctrl + Alt + F7 will bring you back to the GUI Login and you can proceed as you would normally.
If using KVM or QEMU Virtual Machines please use 'virtio' as your video adapter
If using Virtual Box please use VMVGA as your video adapter.