Run­ning Cisco VPN on OS-X I would get “Error 51: Unable to com­mu­ni­cate with the VPN sub­sys­tem” when­ever I would try to con­nect. I tried reboot­ing, mov­ing from Wire­less to Wired (and reboot­ing in between), and noth­ing would work. However,

sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart

seems to fix the issue.