Think I found what is going wrong: in BIOS I had the setting "USB Support" under the BOOT-section set to DISABLED. When changing this to PARTIAL INITIALIZATION, the problems seem to disappear.
I've rebooted my system now for about 10 times, and all going fine.
Can someone of the ROG-guru's indicate what the impact of this setting is, and whether this according to them could lead to the behavior I'm observing?
Thx