SOLVED, see my reply below.
I have two new Linux 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.
Given that this happens when trying to save settings in the motherboard, I suspect that's where the problem lies.
(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 an Asus ROG Crosshair Hero VIII (WiFi); Linux kernel 5.11.0-37, AMD Ryzen 7 3800XT CPU; the GPU is aGigabyte GeForce RTX 3060, Nvidia driver 470.63.01.
Would anyone have a clue as to where I might start troubleshooting for this?
Thanks in advance, I can provide more info re my system if needed.