Error 51: Unable to communicate with the VPN subsystem.
Please make sure that you have at least one network interface that is currently active and has an IP address and start this application again.

Problem

Snow Leopard operates in 32 bit mode by default, Lion is a 64 bit operating system. Cisco’s VPN client only runs in 32bit mode.

Fix

Don’t try to restart your Cisco client, it doesn’t work.

Example:

$ sudo SystemStarter restart CiscoVPN
Password:
Stopping Cisco Systems VPN Driver
(kernel) Kext com.cisco.nke.ipsec not found for unload request.
Failed to unload com.cisco.nke.ipsec - (libkern/kext) not found.
Starting Cisco Systems VPN Driver
/System/Library/Extensions/CiscoVPN.kext failed to load - (libkern/kext) requested architecture/executable not found; check the system/kernel logs for errors or try kextutil(8).

All you need to do is start Lion in 32bit mode. You can simply do this by holding down the 3 and 2 key when booting up.

It’s not the best solution but it is a temporary solution until Cisco comes out with a 64bit compatible client.

Update

Switching between Lion 64-bit and 32-bit

To check which mode you are currently in, run this command on the terminal:

$ systemsetup -getkernelbootarchitecturesetting

To set your machine to boot into 64-bit mode, enter this command and reboot:

$ sudo systemsetup -setkernelbootarchitecture x86_64

To set your machine to boot into 32-bit mode, enter this command and reboot:

$ sudo systemsetup -setkernelbootarchitecture i386

To set your machine to boot into default mode, enter this command and reboot:

$ sudo systemsetup -setkernelbootarchitecture default

Your settings are updated in /Library/Preferences/SystemConfiguration/com.apple.Boot.plist