Win2000 – error 691

I have created a dial-up connection in Win2000 to a secure site. After dial-up makes the connection the user must enter the correct username / password before they can access content.The problem is not the dial-up login but the second authentication brings up the error message as follows: eror 691 – username or password is invalid on the domain. The same username / password works fine from a Win 98 PC.