Occasionally, I’ve gotten the following error when trying to launch the Cisco VPN Client:

Error 51 : Unable to communicate with the VPN Subsystem. Please make sure that you have at least one network interface that is currently actove and has an IP address and start this application again.

Rebooting usually fixed the problem. I finally got annoyed with having to reboot, and found a faster solution on MacInTouch’s Tiger Incompatiblity page (and yes, I’m already running Leopard):

Seems like unloading and reloading the kernel module fixes it.