Based on the error (and the fact the address is 127.0.0.1) can tell that the client on a relay machine is attempting to post a report to its local relay component.
First, try restarting the relay service on this machine, then try restarting the client service. Did that solve the issue?
Is the relay overloaded with clients? How many clients have you assigned to this relay?
What version of BigFix for the client and relay components are you running on this relay machine?