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, [...]