somehow the problem solved itself. I restarted the comptuer a few times, tried again with activate, and so on. No changes in DNS/files were made.
After a few UPDATE/restarts tries, the licence got automagically updated (I could see the new expiration date), but the red ACTIVATE button was still there. After a couple of minutes or so, Avast got out from the Subscriptions menu I was looking at, and the ACTIVATE button became UPGRADE. Now I can correctly see the new license (expiring on the 16th of February, in 366 days). Also, the old license file was from the 17th of September.