$.ajax call returns net::ERR_CONNECTION_RESET wcf service

Question

I have an ajax POST call to my wcf service. It was working fine when I was returning a string (json payload) but as soon as I switched to returning an 'HttpResponseMessage' object it started giving me the ERR_CONNECTION_RESET. Same payload returned with the exception of now I want to return proper responses like 200 and 404's.

Here is the ajax call:

function Login (cred) {
$.ajax({
    type: "POST",
    url:  ServerConfig.Server + ServerConfig.Service + "/login/",
    // The key needs to match your method's input parameter (case-sensitive).
    data: JSON.stringify(cred),
    contentType: "application/json",
    dataType: "json",
    crossDomain: true,
    beforeSend: function(xhr) {
        xhr.setRequestHeader("Access-Control-Allow-Origin", "*");
    },        
    error: loginFailed,
    success: loginSuccess
});
}

and here is the my wcf method:

public HttpResponseMessage Login(string username, string password)
{
        repository = new DbRepository();
        HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.BadRequest);
        var res = string.Empty;

        var user = repository.Login(username, password);

        if (null != user)
        {
            res = JsonConvert.SerializeObject(user);
            response = new HttpResponseMessage(HttpStatusCode.OK)
            {
               Content = new StringContent(res)
            };
        }
        else
        {
            return response;
        }            

        return response;
}

I have confirmed that by switching back to string works as expected. Can someone shed some light on what might be going on?

Thanks.

EDIT: Here is what my 'cred' object looks like and how it was created:

$('.signin').on("click", function () {
            var user = $('.username').val();
            var pass = $('.password').val();
            var cred = { "username": user, "password": pass };
            Login(cred);
        });

The JSON.stringify(cred) looks like this:

{"username":"Test1234","password":"TestPassword!"}