Well I don't really know what to suggest, given that this is happening across different computers and different builds.
Do you have the same firewall in all systems (and if so what) ?
Though that shouldn't be an issue as you say that they all appear to be up to date.
Try a repair of avast on one of them and see if that makes a difference. Though again it doesn't account for why it has to all of your systems and different builds.
From windows Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.