avast doesn't block but scans and alerts to infection, are you saying that is what it does ?
Are there any errors displayed, if so what ?
Rather than disable all of avast, try just disabling one shield at a time to see which is is. I don't believe the P2P Shield would be monitoring this activity as it wouldn't be on its supported p2p applications list, so try that first.
Try the Web Shield next as that monitors port 80 HTTP traffic, if the game uses port 80 then the web shield would expect the HTTP protocol to be used and if a P2P protocol were used that could tie up the web shield.
Try the Network Shield next, though I don't believe that would have any impact unless it considers where the map is being downloaded from is considered malicious, in which case you would get an alert.
Sorry I can't be any more practical help I'm not a gamer.