A little bit more help from Avast would be nice.
Maybe a little late but here are my two ways of fixing the Problem:
On my laptop I started windows in safe mode, uninstalled Avast free, boot in normal mode, reinstall and reconfigure Avast completely.
On my PC I also started windows into safe mode with network! First I tried to start Avast and make a normal update. Unfortunately it did not work.
Then I went to control panel -> Programs and Function, selected Avast Pro entry and hit the Uninstall/Change button.
In the uninstallation wizard I discovered the "Update" Tab and tried that before uninstall. This update completed successfully and I was able to restart my PC in normal mode without any further action.