Is there a clear path between the ACE/Server and the SG/SDI server?
Its possible that since SDI is a UDP-based protocol that it is not being properly passed back to the ACE/Server from the SG/SDI Server. Use the traceroute utility to determine if there are additional hops between the two. These hops may be filtering UDP requests and should be opened to pass UDP on ports 5500 and 5510 at the bare minimum.