VPN Error Code 789

Hi,

I'm unable to connect to my Allied Telesyn AR-410s from a remote spot with
Vista.
It's a VPN gateway/router at home. With XP this worked perfectly.
I'm using L2TP-Ipsec and a Preshared key.
Just can't get it working in Vista RTM Build 6000.
It just gives the error code 789.
Anyone experiencing the same? Perhaps someone with a solution?

Thanks in advance,
Wesley