JSON response contains XML header

I'm sending a HTTP request to an ASP.NET web service and it returns the response as JSON (The web service originally returns a DataTable and it uses JSON.NET to convert it to JSON).

I'm getting the response as JSON. But with that an XML header comes along!

2013-02-21 16:42:41.062 HttpRequestTest[6023:c07] <?xml version="1.0" encoding="utf-8"?>
<string xmlns="http://www.my-comp.com/">[
  {
    "Int_Proj": 152,
    "NewMailCnt": 79,
    "DocsForRev": 2,
    "DocsForRel": 1,
    "Int_Sec": 2,
    "UnregCnt": 3225,
    "UnregAccss": 3,
    "OutstMLCnt": 276,
    "TBEdition": 1,
    "ProjNo": "TESTPROJ1",
    "ProjTitle": "TESTPROJ1 - DO NOT TOUCH TESTPROJ1 - DO NOT TOUCH TESTPROJ1",
    "ServerName": "ISURU",
    "ServerUrl": "ISURU/myserver/",
    "Int_Server": 1,
    "Int_Key": 2,
    "IsGblAdrBk": 0
  },
  {
    "Int_Proj": 160,
    "NewMailCnt": 11,
...

Here's the Objective-C code for calling the web service.

NSURL *url = [NSURL URLWithString:@"http://isuru/TBWebService/Session.asmx/GetUserProjectList?userID=dr&companyID=qas&password=david&errorMsg=''"];

NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url
                                                cachePolicy:NSURLRequestReturnCacheDataElseLoad
                                            timeoutInterval:30];
NSData *urlData;
NSURLResponse *response;
NSError *error;

urlData = [NSURLConnection sendSynchronousRequest:urlRequest
                                    returningResponse:&response
                                                error:&error];
NSString *returnedData = [[NSString alloc] initWithData:urlData
                                                   encoding:NSUTF8StringEncoding];
NSLog(@"%@", returnedData);

And here's the C# code of the ASP.NET web service method.

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public string Logon(string userID, string companyID, string password, string projNo)
{
    MySoftware.TestSoft.User user = new MySoftware.TestSoft.User();
    DataTable userTable = user.ListUserAccessProjects(userID, companyID, password, "", "");
    if (userTable.Rows.Count == 0)
    {
        string jsonRet = JsonConvert.SerializeObject("ERROR - Invalid User ID, Company ID or Password.", Formatting.Indented);
        return jsonRet;
    }

    DataRow[] projectEntries = userTable.Select(String.Format("ProjNo='{0}'", projNo));
    if(projectEntries.Length==0)
    {
        string jsonRet = JsonConvert.SerializeObject("ERROR - Invalid Project Number.", Formatting.Indented);
        return jsonRet;
    }

    int int_Project = Convert.ToInt32(projectEntries[0]["int_proj"]);

    MySoftware.TestSoft.Logon logon = new  MySoftware.TestSoft.Logon();
    try
    {
        bool isSameServer = false;
        string sessonKey = logon.Login(Convert.ToInt32(projectEntries[0]["int_Key"]), Convert.ToInt32(projectEntries[0]["Int_Server"]), userID, companyID, password, int_Project, out isSameServer);
        string json = JsonConvert.SerializeObject(sessonKey, Formatting.Indented);
        return json;
    }
    catch (ApplicationException ex)
    {
        string jsonRet = JsonConvert.SerializeObject("ERROR - " + ex.Message, Formatting.Indented);
        return jsonRet;
    }
}

This same issue has been asked here on SO and I tried everything mentioned there but the to no avail. Besides none of them was related to Objective-C.

How can I get the JSON response without the XML header?