The Cisco VPN client occasionally shows an error dialog on a mac (Error 51: Unable to communicate with the VPN subsystem) when I attempt to start it, when the internet is working just fine. I found the fix in the following link which worked like a charm.
Specifically, this one line command did the trick for me:
$ sudo kextload /System/Library/Extensions/CiscoVPN.kext
#1 by B. Campbell on September 3, 2010 - 6:21 am
There may be more you need to do. You can also try this if it still doesn’t work.
http://forum.maas360.com/go/mobileitexpertise/fix-for-cisco-vpn-error-51-unable-to-communicate-with-the-vpn-subsystem/
#2 by Torben on May 24, 2011 - 2:33 am
Did the trick for me too. Thanks a lot.