I had a client email me with basically the same problem today. Either it returns a rate or it just never seems to return anything. This was with the advanced shipper mod though. Turning debug on only returned something when it returned rates. I don't have access to the server on this account so I don't know if it's been writing any debug logs. I tried to recreate the problem on a local WAMP install and never did get it to fail. That tells me that there is either something with the ZC install, server config, or just dumb luck when I tested it locally.