Occasionally when starting Cisco's VPN client on Mac OS X I am greeted with the following popup window stating - Error 51: Unable to communicate with the VPN subsystem. I fix this issue, which seems to be related to wireless adapters disappearing and reappearing by typing the following command in the terminal window, hitting enter and entering your password. If you do not always use the terminal like myself, you can find the application in Applications -> Utilities.
sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
You should see something like the following.
scotbuff@growler~$ sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
Password: *********
Stopping Cisco Systems VPN Driver
kextunload: unload kext /System/Library/Extensions/CiscoVPN.kext succeeded
Starting Cisco Systems VPN Driver
kextload: /System/Library/Extensions/CiscoVPN.kext loaded successfully
scotbuff@growler~$Afterwards I can always successfully connect. I noticed this issue with Tiger and it still appears now and then in Leopard and can be fixed the same way.
