The Cisco VPN client 4.7.00 (0510) mostly works for me on OS X 10.4.4, but I get “Error 51” sporadically and I used to have to reboot to stop it, but now I took a tip that I found on VersionTracker and did a slight tweak to the command so that the cvpnd process daemonizes silently to the background and you don’t have to remember to keep restarting it:
launchd -d /bin/sh -c \ "while [ 1 ]; do /private/opt/cisco-vpnclient/bin/cvpnd; done"
Although hopefully Cisco will solve the problem so that we don’t have to resort to hacky solutions like this.
This tip was just posted on macosxhints:
http://www.macosxhints.com/article.php?story=2006021411494452
This is what I did to fix this problem:
http://forum.maas360.com/go/mobileitexpertise/fix-for-cisco-vpn-error-51-unable-to-communicate-with-the-vpn-subsystem/