ERROR: THE RPC_S_SERVER_UNAVAILABLE ERROR (OX6BA) WAS THROWN BY THE RPC RUNTIME PROCESS
Setting the scene - Exchange Server 2007 (as part of SBS 2008)
I checked everything - the SSL certificate and bindings in IIS, checked the RPCProxy.dll opened with blank page externally, checked all of the allowed ports in the registry but could not get past this error. Using www.testexchangeconnectivity.com this error occurred at the point: Attempting to ping RPC endpoint 6001 (Exchange Information Store) on server. The attempt to ping the endpoint failed.
In the end, the resolution was to either disable IPv6 (not my chosen method) OR to add the IPv4 addresses into the hosts file on the CAS which then fixed the problem. Who would have thought it!
Credit to Yusuf Panjri for the fix. His very helpful article can be found here. Many thanks to him for this.
Hope this helps others out there.