I bought Snow Leopard (Mac OS 10.6) and installed in on one of my machines. I am performing an installation on my MacBook, and to while away time, decided to check my mails via Entourage. When I started Shimo for establishing my VPN connection, I got the following error:
Error 51: Unable to communicate with the VPN subsystem. Please make sure that you have at least one network interface that is currently active and has an IP address and start this application again.
I had similar problems with the Cisco VPN when I installed Mac OS 10.5 (Leopard), and so was not surprised when the VPN Client reared its ugly head again. I knew that it would be foolish to wait for Cisco to upgrade their VPN client, and started thinking of ways to resolve this problem.
Since Snow Leopard is an upgrade, it would mean that the certificate or authentication might have gotten screwed in the process, and so, I uninstalled the VPN Client (using AppZapper), and then reinstalled it. And that was it! The VPN client connected smoothly and I am connected to my exchange server via Entourage and Shimo!
PS: Snow Leopard is sleek and pretty fast, I must say! Will tinker some more and post more about my findings.