Do you still have your original email with the license number, if so then this is what you need to do - avast! 5 Pro license convert from 4.8 Pro license. Try http://www.avast.com/resend-license.php now, fill in the form giving your email and 4.8 Pro License key and they will send the new license file to you (presumably it will check you have a legit 4.8 Pro license first).
I don’t know why some people are having problems, I have been able to connect every time I tried it. However, I use OpenDNS for my DNS server so I don’t know if the problems some are experiencing are DNS related.
Do your or your ISP use an anti-spam filter as that could be blocking any response, this has happened for some in regard to the receipt of their license email.