I've been quietly lurking, waiting for someone seeing success, and not blindly contributing to a DoS by madly clicking the Update button...
I just tried it, and got the manual update just fine. All my lights are green.
Thanks so much for this forum - and especially to DavidR and mchain for their leadership here!
-Don
PS: Anyone reading this and still wondering what's going on and why the 404, please read earlier comments. It's all there. The problem is with Avast's servers; it's being corrected; you will see results soon. As my Aussie brother say, "No worries, Mate!"
PPS: Avast! ROCKS!!