Error 51: Unable to communicate with the VPN subsystem.When this occurs, I definitely have an active network interface that has an IP address, so it's quite clearly not an issue with my machine's settings. The problem seems to be related to sleeping the Mac after using the Cisco VPN Client; this sometimes (but not always) will lead to the above error message when I next try to use the VPN Client. What makes it extra frustrating is that it's not consistent -- the app will work for days at a time through sleep and wake cycles, then spontaneously decide it's time for error 51 to appear.
Please make sure that you have at least one network interface that is currently active and has an IP address and start this application again.
I used to think rebooting was the only way to solve this problem -- I hadn't had much luck with the solutions posted in this older hint. A while back, after getting "51'd" again, I went in search of a better solution -- and I found one. In Terminal, use this relatively simple command to stop and restart the kernel extension that the Cisco VPN Client uses:
sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
In the two or three times I've had this problem occur since finding the above solution, restarting the kernel extension has fixed the problem every time. So thanks to Anders.com for solving this frustrating issue with the Cisco VPN Client. I would hope a future Cisco VPN Client update would solve the issue entirely, but that fix has been published for two years already, so I'm not holding my breath.

