C# .NET HttpWebRequest.GetResponse() gives SendFailure to EC2 behind ELB over https

Our .NET code is posting using JSON to an HTTPS REST API. It works on machines from Vista up (I think they all have .NET 4.5), however, we need to support down to Win XP with .Net 4.0 and on that machine our code fails on the last line shown below.

If we change the URI to HTTP from HTTPS, it works fine. A number of the lines you see below (such those for CertificateValidationCallBack) were added in an attempt to fix this issue).

BIG FLAG: We have noticed that on this XP machine, IE 8 gives "Internet Explorer cannot display the webpage" over HTTPS, though HTTP is fine. We added the CertificateValidationCallBack handler hoping it would get around this issue. I'm not sure it does.

Some information about the server configuration. It is an EC2 instance behind an ELB. The ELB holds the SSL cert and sends data received from port 443 to port 80. Apache on the EC2 instance just deals with port 80.

    private static bool CertificateValidationCallBack(
        object sender,
        System.Security.Cryptography.X509Certificates.X509Certificate certificate,
        System.Security.Cryptography.X509Certificates.X509Chain chain,
        System.Net.Security.SslPolicyErrors sslPolicyErrors)
    {
        return true;
    }

     HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(uri);
        webRequest.PreAuthenticate = true;

        using (var client = new WebClient())
        {
            client.Headers.Add(HttpRequestHeader.ContentType, "application/json");
            try
            {
                string responseString ;
                ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls;
                ServicePointManager.ServerCertificateValidationCallback = CertificateValidationCallBack;
                _webEx = null;
                webRequest.ContentType = "application/json";
                webRequest.Method = "POST";
                byte[] bytes = System.Text.Encoding.ASCII.GetBytes(credsString);

// webRequest.ContentLength = bytes.Length; webRequest.KeepAlive = false; webRequest.Timeout = -1; System.IO.Stream os = webRequest.GetRequestStream(); os.Write(bytes, 0, bytes.Length); //Push it out there os.Close(); System.Net.WebResponse resp = webRequest.GetResponse(); ... } }