Hello, if you are searching for reasons and fixes as to why your automatic updates do not work, but your manual updates do, I have yet another solution. I spent many many hours of research on the forums here, and also had many helpful emails suggesting every fix even remotely possible, from the avast tech support. In the end, it turned out to be something that I had suspected, but didn't manage to thoroughly check out until every other avenue had been exhausted. Including me.
The fix in my case turned out to be in my router settings. I had updated the firmware many months ago, and although it seemed to turn out a bit weird, everything seemed to work. (ie, internet explorer, firefox, other antivirus and antispyware programs, outlook express, etc.) That is, until months later I realized that my avast was not getting automatically updated anymore. To make a long story shorter, it had to do with the internal time and date settings in the router itself. After reflashing the router firmware with another version, i was finally able to make the correct settings stick, and voila, avast was able to auto update again. So obviously the incorrect date and time setting was throwing off avast somehow.
So the moral of the story is, if you are having issues with avast automatic updates, check your router time and date settings in addition to any other fixes. Cheers.