The ISA Server RPC filter blocks RPC traffic after Windows Server 2003 Service Pack 1
is installed on a computer that is running ISA Server 2004 or ISA Server 2000
You install Microsoft Windows Server 2003 Service Pack 1 (SP1) on a computer that is running Microsoft Internet Security and Acceleration (ISA) Server 2004, Standard Edition or Microsoft Internet Security and Acceleration (ISA) Server 2000. After you install the service pack, the ISA Server remote procedure call (RPC) filter blocks RPC traffic between networks.
|•||RPC traffic between all other servers is not affected.|
This problem does not occur with ISA Server 2004, Enterprise Edition
For the rest of the details, check out KB 88722 at http://support.microsoft.com/?id=887222
Thanks to Jim Harrison for the heads-up on this valuable KB offering!