From which side do you try to run the VPN connection? If it is from WAN side make sure you aren't behind a home router NAT or any other such device.
Usually error 412 means that
- something between you and ASA blocked your connection (ex IKE protocol may not be passed through NAT correctly).
- Poor network connection. Packet delay or loss can force the VPN session to drop
- Hardware failure in a network interface card or network device.
The first one is the most certain. Try connecting from a public IP (cable or dsl modem plugged directly).
Cheers