I’ve had a few problems with the Cisco VPN Client for Mac OS X (release 4.9.01 (100) ) recently.
I’d get intermittent “Error 51: Unable to contact the VPN subsystem” when opening the VPNClient appliaction.
Some times a reboot would fix it, sometimes it wouldn’t.
I can’t reliably reproduce it either, sometimes coming out of sleep breaks it, sometimes restarting breaks it.
I think I’ve worked out how to resolve this now from a command line:
Firstly take the fw0 interface down and bring it up again:
Sudo ifconfig fw0 down
Sudo ifconfig fw0 up
Then restart the VPN SS:
Sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
That has resolved it for me the last 5 times it happened, without needing to reboot again.
Hope that helps someone else out.