None of this helps, much is too invasive to try.
The issue seems related to multimonitor. Its an ASUS FX705gm with Nvidia graphics and integrated Intel graphics.
I have a 4K monitor plugged into the external HDMI port. Its setup with the NVidia X Server Settings tool.
But the Ubuntu-Mate Display control panel is also active and required to make the 4K display be "primary" The builtin laptop LCD can be active or not. It seems to run on the Integrated graphics if the 4K display is attached.
Normally I have the LCD disabled and run with the lid closed. It doesn't matter if the LCD is active or not I get stuck in the login loop when I try to reboot.
What I tried after today's reboot required updates was to open the laptop lid, unplug the hdmi monitor, which forced the display back to the laptop LCD and reactivated it, aparently using the NVidia driver. Reboot from there worked normally. Plugging in the external monitor again at the bios screen had it boot back to the 4K display as primary with the LCD disabled.
PITA, but better than pulling the plug to reboot! I hooked up and old "passive" hdmi switch to avoid wear and tear on the laptop hdmi port.