Hi,
One common issue is firewall software blocking the ports in GhostCast server. However, we can rule it out since the client can connect to the server.
Then it most likely multicasting not available in the network, but you could multicast successfully with the backup server.
In large networks, usually servers are in a different subnet with more restrictions. Does your backup server is also in the same subnet as the main server? Could it be multicasting not available in the subnet of the server?
Krish