VPN & MS Proxy server error.

I’m trying to make a vpn connection to my NT 4.0 server(MS Proxy 2.0 installed). I’ve added PPTP protocol on NT, configured RAS to accept calls on the vpn port. I added the VPN adapter to my windows 98 workstation and dialed into my ISP, I can ping the NT server, however when I try the VPN connection i get the following error: “The Microsoft Dial-Up Adapter is in use or not responding properly.Disconnect other connections and then try again.If this problem persists, shut down and restart your computer.”

Any suggestions ?