I was getting annoyed today by my Cisco VPN client unwilling to do its job. As I had been playing around with my Time Capsule and my wireless accesspoint to get my wireless network straight so that it could connect with my Nintendo Wii, I encountered an 'Error 51'. Luckily I wasn't the only one who got that problem. The first solution already worked:"The simple fix is to quit VPNClient, open a Terminal window, (Applications -> Utilities -> Terminal) and type the following:

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

(for OS X 10.5 and lower) or

sudo kextload /System/Library/Extensions/CiscoVPN.kext

(10.6) and give your password when it asks. This will stop and start the "VPN Subsystem", or in other words restart the CiscoVPN.kext extension. Cisco seems to have problems when network adapters disappear and reappear, something that happens commonly in Wireless or Dial-up scenerios. Sometimes putting a system to sleep, disconnecting an Ethernet cable or simply reconnecting your wireless will cause CiscoVPN to loose track of the network adapters on the system."The source can be found here.