Thanks Joerg for the Response !!
When we saw this issue, we found the Proxy setting was Enabled due to some XYZ reason(not sure), but it start failing the request of replication as TMG settings(who does proxy policy settings) they changed the policy so all the request going to replication server was getting failed.
So what change in configuration we have done is,we added the server list in No proxy list which gave Test Connection successful for the replication.
So Joerg i want to know 2 things now:
=>Proxy setting policy setting is something which is out of our scope??TMG is vendor who does that change.
=>what are the list of issues majorly occurs in Replication?