Hmm, this is most strange.
The server had malfunctioned for some days when I wrote my question. Being away for some days, and today I picked the server out of the closet to do some investigations as suggested above. Power on - and everything works....
So I seem to have a self healing server, which in one way is very nice, but I hate to have these situations when I cannot find the cause of the fault. Most likely it'll come back again.
Thanks for you good suggestions, I'll save then until it's needed again. ;-)