httpClient.PostAsync works on Android and iPhone simulator but fails on real iPhone or iPad

Hi, I recently got caught up on XCode and Xamarin updates. Now a Post from a real iPhone device gives me a 500 error while the same code in a Droid phone or an iPhone Simulator from Visual Studio works fine. Any ideas on how to fix this or investigate it further are appreciated.

`

   private async Task<string> GetClientInfo(string ClientID)
    {
        string url = "";

        Uri resourceAddress = new Uri("https://blahblah");
        var values = new Dictionary<string, string>
        {
            { "clientid", ClientID }
        };
        var content = new FormUrlEncodedContent(values);
        HttpResponseMessage response = new HttpResponseMessage();
        string clientID = "";
        HttpClient httpClient = new HttpClient();
        try
        {
    //////////////////////////////////////////////////////////////
    // The following line is where it fails in an iPhone device
            response = await httpClient.PostAsync(resourceAddress, content);
            if (response.StatusCode == HttpStatusCode.OK)
            {
                string responseString = await response.Content.ReadAsStringAsync();
                Dictionary<string, object> theData = JsonConvert.DeserializeObject<Dictionary<string, object>>(responseString, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore });
                var results = theData["results"].ToString();
                if (theData["client"] != null)
                {
                    var client = theData["client"].ToString();
                    Dictionary<string, object> theData2 = JsonConvert.DeserializeObject<Dictionary<string, object>>(client, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore });

                    clientID = theData2["clientid"].ToString();
                    url = theData2["url"].ToString();
                }
                else
                {
                    string msg = string.Format("ClientID {0} was not found.", ClientID);
                    Debug.WriteLine(msg);
                    WarningMsg = msg;
                    return url;
                }
            }
            else
            {  // Post failed
                string msg = "LogInView GetClientInfo: Failed accessing web server (Post) for clientid " + ClientID + " Msg: " + response;
                Util.DisplayErrors(msg, WarningMsg);
                WarningMsg = msg;
            }

        }
        catch (Exception ex)
        {
            string msg = "\n   ***   LogInView GetClientInfo: Exception when accessing web server (Post) for clientid " + ClientID + " Msg: " + ex.Message;
            Util.DisplayErrors(msg, WarningMsg);
            WarningMsg = msg;
            return "";
        }
        return url;
    }

`

Here are the results for different devices and simulators taken at the line where it fails for an iPhone device:

iOS iPhone and/or iPad:
{StatusCode: 500, ReasonPhrase: 'Internal Server Error', Version: 1.1, Content: System.Net.Http.NSUrlSessionHandler+NSUrlSessionDataTaskStreamContent, Headers:
{
X-Powered-By: ASP.NET
Server: Microsoft-IIS/8.0
Date: Tue, 14 Mar 2017 16:50:22 GMT
X-Powered-By: ASP.NET
Content-Type: text/html
Content-Length: 75
}}

iPhone Simulator (iPhone 4s iOS 9.3) or (iPhone7 – iOS 10.2) run via Visual Studio:
{StatusCode: 200, ReasonPhrase: 'OK', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
{
Server: Microsoft-IIS/8.0
X-Powered-By: PHP/5.6.30, ASP.NET
Date: Tue, 14 Mar 2017 17:54:38 GMT
Content-Length: 129
Content-Type: application/json
}}

Android phone:
{StatusCode: 200, ReasonPhrase: 'OK', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
{
Server: Microsoft-IIS/8.0
X-Powered-By: PHP/5.6.30, ASP.NET
Date: Tue, 14 Mar 2017 17:51:03 GMT
Content-Length: 129
Content-Type: application/json
}}