New 20.04 install hangs on re-boot

I have two new 20.04.3 installs (Ubuntu-MATE) on a newly built desktop workstation. One is on a 2.5-inch SSD, the other on an nvme SSD.

If I do a "Restart" from either, it appears as if a restart will happen, but it just hangs.

The computer does not restart, does not shut down, it just goes black, and I have to press the reset button.

The same thing happens if I make any change in the UEFI/BIOS settings. When I save the changes, there is no shutdown and no reboot, just a hang, and I have to press the reset button.

(The motherboard's Q-codes, after quickly flashing through a bunch of numbers & letters, just go back to "AA" during the attempted reboot, indicating nothing useful.)

The motherboard is Asus ROG Crosshair Hero VIII (WiFi); kernel 5.11.0-37, AMD Ryzen 7 3800XT CPU; GPU is Gigabyte GeForce RTX 3060, Nvidia driver 470.63.01.

Would anyone have a clue as to where I might start troubleshooting for this?

I'll provide more info re my system if needed.

Solution 1:

SOLVED: I removed the CMOS battery on the motherboard, waited one minute, and put it back in. Problem solved.