Every now and again, I get the following message when starting the Cisco VPN Client on my iMac:
Error 51: Unable to communicate with the VPN subsystem
It turns out there is an easy way to fix the error. Simply open a Terminal window and type the following:
sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
It will ask you to enter your password then it will restart the VPN subsystem and you should then be able to run the VPN Client with no more errors (once or twice I’ve had to run this command a second time before it would work).