I was logged off suddenly all the time too, until I started ticking off the "Remember me"-box when logging in. Afterwards it was better. Have you tried that? Didn't help with today's troubles, though.
I hope you're feeling better today.
I've tried that on innumerable occasions Markus and it's made no difference whatsoever. At least with reading about similar problems others are having, I know it's not my dino pc now.