Problems addressing a webservice

Guys,

I have the following code:
public async Task GetToken(string username, string password)
{
using (var client = new HttpClient())
{
client.BaseAddress = new Uri(this.servername);
client.DefaultRequestHeaders.Clear();
client.Timeout = new TimeSpan(0, 1, 30);
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            try
            {
                string webservice = string.Format("{0}/Webservices/FormAuthentication.svc/REST/Login?username={1}&password={2}", this.methodBeginning, username, password);
                var response = await client.GetAsync(webservice);
                if (response.IsSuccessStatusCode)
                {
                    var result = response.Content.ReadAsStringAsync().Result;
                    return result.Replace("\"", string.Empty);
                }
            }
            catch (Exception ex)
            {

            }

            return string.Empty;
        }
    }

On following piece of code, I get this error: 'Attempting to JIT compile method '(wrapper delegate-invoke) :invoke_callvirt_void_HttpWebRequest_bool (System.Net.HttpWebRequest,bool)' while running with --aot-only'

var response = await client.GetAsync(webservice);

Works fine in Android/Phone, on iOS Simulator, but not on my iPhone 5s?

Searched for hours at the internet but no conclusive answer, can please somebody help me?

Thanks!
Alain.