That is the first suspect, as Rich says. Suspect #2 is whatever security you have. It is quite possible for your firewall to allow communication, but your AV software treating the cow activity as a port sniffing, and block the main box activity (rare, but it can happen). If they are the same build number, and there is no comm blockage, then most likely one cow had a blown update that broke it. The only fix there is to completely clean out the cow on the compromised unit and reinstall it....