There was a problem on the engine update around 2012JAN24 - 2012JAN25. After a new VPS was released, by manually updating avast http://www.avast.com/download-update, rebooting, and maybe some additional “turn on” shields if necessary, you would had solved the problem.
By uninstalling and reinstalling (and then updating the VPS to the corrected date) you in fact got almost the same effect.
The QA procedures were improved after this incident, so let’s hope it doesn’t happen again.