Hmmm...
Turn off "Failsafe Watchdog" in the Performance tab
interesting...
I just tried it and at least my system boots now (cold and warm), with the caveat that the boot process hangs at the "Intel NUC" logo for about 5 seconds for every USB3.0 device currently attached.
e.g.:
0 devices: 2 seconds
1 device: 6 seconds
2 devices: 12 seconds
3 devices: 17 seconds
4 devices: 25 seconds
This doesn't make me feel very confident, especially since I have turned off booting from USB (all boot devices checkboxes are off!). I would expect the BIOS to ignore the USB devices completely - obviously it doesn't.
As for turning off the watchdog, according to the BIOS glossary:
When the failsafe watchdog is enabled, after a boot failure, the system will reboot back into BIOS Setup with the last values set by the user.
So, "playing it safe with crashes, causes the BIOS to crash"?!?
Thanks for the TIP - at least I can continue experimenting now :-)