Check the service “avast iAVS Control Service”.
Control Panel → Administrative Tools → Services.

Open its properties and go to the Log On tab. What account is it set to run under? The Local System?

If so, change this account to an admin account with outbound HTTP access allowed. E.g. the same account that was specified during avast install (this is the account that should now be in the Log On property of the “avast! Antivirus” service).

After making the changes, restart the “iavs4 control service” and retry the manual update. Hopefully, it will now work.

Thanks
Vlk