Cisco VPN Client – Error 51: Unable to communicate with the VPN subsystem
This will sometimes occur after software updates, if you do not reboot your computer semi-regularly,
or if one or more VPN dependent services on your computer have stopped responding.
Try typing one of the following lines into Terminal (depending on your version of OSX, one of these may or may not work on your computer):
sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
sudo kextload /System/Library/Extensions/CiscoVPN.kext
sudo SystemStarter restart CiscoVPN
If the problem persists, try running a Disk Permissions Repair next.
Go to Applications > Utilities > Disk Utility, select your Macintosh HD, choose the First Aid tab, and click the Repair Disk Permissions button in that window.
If this still does not solve your problem, and if you have the original installer or disk image .dmg file, try uninstalling and re-installing the VPN Client.
You may need to (carefully) delete Cisco VPN app from /Applications/ as well as related files and folders inside of /Library/Application Support/, /Library/LaunchDaemons/, /Library/LaunchAgents/, /Library/Preferences/, /Library/Caches/ as well as the previous folders inside of /Users/username/Library/Application Support/, etc…