Strange RPC Error on Windows Server 2008

Hi all,

I have a very strange problem when connecting to a remote server using programs such as Event Viewer, DNS, DCHP etc. Basically we have two seperate sites (same subnet) connected via a VPN link between an ISA 2004 server and a Dreytek 2955 and both sites have a mixture of Windows 2003 and 2008 servers.
In this example I will use Event Viewer as my test program and will try to connect to another server event log. The results I get are as follow:

Test One
Setup: Connect to a Windows 2008 Server on Site B from Site A using Windows Server 2003
Result: OK

Test Two
Setup: Connect to a Windows 2003 Server on Site B from Site A using Windows Server 2003
Result: OK

Test Three
Setup: Connect to a Windows 2003 Server on Site B from Site A using Windows Server 2008
Result: OK

Test Four
Setup: Connect to a Windows 2008 Server on Site A from Site A using Windows Server 2008
Result: OK

Test Five
Setup: Connect to a Windows 2008 Server on Site B from Site B using Windows Server 2008
Result: OK

Test Six
Setup: Connect to a Windows 2008 Server on Site B from Site A using Windows Server 2008
Result: Error (The Remote Procedure Call Failed)

Test Seven
Setup: Connect to a Windows 2008 Server on Site A from Site B using Windows Server 2008
Result: Error (The Remote Procedure Call Failed)

Therefore can anyone shed any light on why we are getting the RPC error when going from Windows 2008 to Window 2008 server across the VPN?

Thanks
Anthony