If you are running Cisco’s VPNClient on Mac OSX, you might be familiar with (or tormented by) “Error 51: Unable to communicate with the VPN subsystem
“. The simple fix is to quit VPNClient, open a Terminal window, (Applications -> Utilities -> Terminal) and type one of the following:
(for older versions)
sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
(for newer versions)
sudo kextload /System/Library/Extensions/CiscoVPN.kext
and give your password when it asks.
Original article from: http://anders.com/cms/192/CiscoVPN/Error.51:.Unable.to.communicate.with