从ajax调用本地C#Web服务，401错误未经授权

我有一个由ajax调用的登录函数的问题，这是该函数的代码，它是一个Web服务可见

[WebMethod]
    [System.Web.Script.Services.ScriptMethod]
    public string CheckData(string login, string Pass)
    {
        global gb = new global();
        if (gb.CheckUserExist(login, Pass))
        {
            System.Web.Security.FormsAuthentication.RedirectFromLoginPage(login, true);
            HttpCookie cookie = new HttpCookie("userData",login);
            cookie.Expires = DateTime.Now.AddMonths(2);
            Mosab2aModel.Mosab2aEntities context = new Mosab2aModel.Mosab2aEntities();
            var User = context.Users.Where(x => x.UserName == login && x.Password == Pass)
                .Select(x => new { x.UserName, x.Password, x.Admin, x.DisplayName, x.FBID }).First();
            cookie["UserName"] = User.UserName;
            cookie["Password"] = User.Password;
            cookie["isAdmin"] = User.Admin.ToString();
            cookie["Name"] = User.DisplayName;
            cookie["FBID"] = User.FBID;
            Context.Response.Cookies.Add(cookie);
            //Context.Response.Redirect("/Default.aspx");
            return "1";
        }
        else
        {
            return "0";
        }
    }

我在这里用ajax称之为

$.ajax({
                    type: "POST",
                    url: 'LoginService.asmx/CheckData',
                    data: "{'login':'"+ login +"','Pass':'"+pass +"'}",
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                        success: function(msg)
                        {
                            if (msg.d == "1")
                            {
                                //document.location.href = '/Default.aspx';
                            }
                            else
                            {
                                formWrapper.clearMessages();
                                displayError('Username or password is incorrect');
                            }
                        },
                        error: function()
                        {
                            formWrapper.clearMessages();
                            displayError('Error in contacting server , try again later.');
                        }
                  });

我有一个奇怪的问题，如果用户不存在，函数运行良好，返回值为0，并显示错误，但如果函数找到用户，则会得到401错误Unauthorized。

编辑：经过几次搜索，问题出在这条线上！

System.Web.Security.FormsAuthentication.RedirectFromLoginPage(login, true);