After you install Windows Server 2003 Service Pack 1 (SP1), you may
experience issues that affect server-to-server communication for TCP/IP
traffic or remote procedure call (RPC) traffic across firewall or
virtual private network (VPN) products in rare and specific network
configurations.
Before carrying any troubleshooting, check the following KBs:
Q899148 Some firewalls may reject network traffic that originates from
Windows Server 2003 Service Pack 1-based computers
Q898060 Installing security update MS05-019 or Windows Server 2003
Service Pack 1 may cause network connectivity between clients and
servers to fail