Hi, sorry for reheating old topic but recently I had similar issue.
My Raspberry Pi 4 model b with 4 Gb Ram got freezes after some periods of time on Ubuntu MATE 20.04
Only mouse pointer was movable and keyboard was mostly unresponsive.
I got it unfrozen by
pressing Ctrl + Alt + F1, then press enter,
enter login and password,
enter 'htop' command,
press F5 for tree view,
with down key find process called xfce4-screensaver,
select 15 SIGTERM, press enter.
After that press 'q' to exit htop, press Ctrl + Alt + F7 to comeback to graphical environment.
Hope that help somebody