We're having the same problem. We had our Avast pop-up telling us the key expited. I have tried for two days to renew it. It won't take the 3 letter code. Then I tried to just re-apply for a new key. Won't take that either, and YES, all the fields were filled out correctly. Didn't have this truble a year a go when we first got Avast and applied for the key. Waht the heck is going on? Until the matter is resolved I can't update my virus protection.
I even sent off an email to support and they didnt help me at all. I gave them info to search for my request and they said they couldn't find my initial attempt at a renew, then told me to try again. While I have tried numerous times to day but yet I still can't do anything.