The same thing has happened to me. I bought Ultimate a few days ago and put the license in and it was fine. Over the weekend I had to reinstall Windows 10, which meant reinstalling Ultimate. Now the activation code doesn't work, and when I clicked to be sent a new license, it doesn't work either.
I sent two emails to Support about this and have had no response. I am on the edge of seeking a refund since Avast doesn't seem to be taking these matters seriously.