Problem solved!
Only I have not a clue what dit it!
I started up a repair went on forever saying nothing and apparently did nothing (no resources in use)
While it was at it, I guess, I rebooted with LAN cable disabled (NO WiFi!)
It did take a while to get shut down. And Windows did seem to have some preparing to do during upstart.
Let Avast search for an updated definition, and it returned! (Saying nothing, like Microsoft, hm?)
Connected the LAN cable again and ran another def. update -> success!