I had the same problem, and searched around for a while and found this fix:
http://www.anders.com/cms/192/CiscoVPN/Error.51:.Unable.to.communicate.with
In short, open a Terminal and enter this command:
sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
It worked for me on the first try.
(thanks a lot, Apple, for wasting one of my telecommute days.)
