update to myself:

I think I found the reason why: I de-installed all the NOKIA Phone software (PC Suite, Nokia Cable Driver etc....) as i want to free up some CPU cycles on my PC... I never connect my Nokia Phone to the PC, and I thought the N800 acts as a standard USB device.
Now it seems to me one does need the NOKIA Cable Driver, as the USB cable for the N800 is the NOKIA DKE-2. I will re-install this driver and report.