Can't register Avast Free 12 (solved!)

My activation will end in a few days and sadly I can’t seem to register again (Avast Free Antivirus 12.1.2272).

If I click the activate button, it just says “Loading…” for about 10 seconds. Then nothing happens - I am not forwarded to any page.

The registration has worked fine previously and I have not recently made any changes to my system (Windows 8.1 64bit). I wonder what’s wrong?

EDIT: The activation worked via another way (a pop-up message reminding me to register).