avast doesn't block, it only scans content and if infected/suspect it would alert you to the problem. avast isn't a firewall so it will have to be something else in the mix that is blocking the upgrade.
Do you happen to have the MS Desktop Search tool ? This has in the caused issues with Outlook when it was indexing the various mailboxes, etc.
As you mention this has been reported in the forums, but was there anything that might resolve it, like uninstall MS Desktop Search, new 'instant search' in Outlook, etc.