If a client PC with Windows XP and OneCare tries to connect via VPN to an ISA-Server 2004, you get exactly this event logged on the ISA-Server. To resolve the problem, you must change the settings of OneCare to enable VPN-Connections.
This event may be recorded when a VPN connection is attempted over a high latency network (i.e. satellite-based Internet connectivity).
From a newsgroup post: "The failure is most likely caused by the hardware router not allowing GRE Protocol 47, which is used in conjunction with PPTP to create VPN between clients or between clients and servers. You may need to allow GRE protocol 47 and TCP port 1723 traffic on your hardware router to enable the connection. See ME241251
for more information about GRE Protocol 47. To resolve the issue, please configure the router to pass GRE Protocol 47 traffic. Some routers call that "PPTP Pass Through" or "VPN Pass Through". Please check the router support pages or the manual for help doing that".
From a newsgroup post: "I had the exact same problem / error after installing SP1 on Windows Server 2003. It broke our VPN connections. Once I uninstalled SP1, the VPN connections worked again. There must be a bug in SP1, or SP1 reconfigures RRAS or the firewall in such a way to prevent VPN connections".