OS X 10.5.1 Upgrade + Cisco VPN + ‘Error 51: Unable to communicate with the VPN subsystem’ ? Try this.

If you’ve recently upgraded to OS X 10.5.1 and noticed your Cisco VPN client reporting a friendly ‘Error 51: Unable to communicate with the VPN subsystem‘ error, you’ll need to do the following.

sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart

Essentially the Cisco VPN extension just needs to be restarted.

Thanks to Anders Brownworth for this tip.