For your information.
Performed a fresh/clean install from the live CD of 20.04 MATE and chose to install third party drivers.
The install went smoothly and I can login. I can confirm with nvidia-settings & nvidia-smi that the nvidia 390.132 driver is correctly installed. System has nvidia GTX675m (Fermi)
Opening just about any window causes instant black screen and the Xorg xserver goes 100% CPU.
Even trying to use the menus on the terminal app causes the black screen.
The only access after this point is via ssh. The Xorg process cannot be killed (-9) and the system indefinitely hangs if a reboot/shutdown is attempted.
The message in the log is the : GPU ... has fallen off the bus
I have submitted nvidia-bug-reports to nvidia dev forum but I see this as a Linux kernel issue as the driver hasn't changed in years. I also don't really see this as a MATE issue.
However thought others may need to be aware if they are having similar issues.
The same system was working fine under 18.04 MATE with driver 390.132 and still works under Windows 10 with same driver (dual boot, separate drive).