Ok, switching Avast to passive mode did the trick! System is running fine again. But that wasn't an easy one, it kept removing the check mark twice before I could make it work, probably due to the Avsat Service still running wild in the background. I then ran an Avast full scan over night to make sure there wasn't malware making Avast Service running wild, but results were perfectly fine, no problems found.
Any hint what I could do next to leave passive mode without running into the same problem again?