Why does HttpWebRequest.GetResponse() fail in an ASP.NET page under IIS?

Why does HttpWebRequest.GetResponse() fail in an ASP.NET page under IIS?

I built a set of API's for one of our developers to consume in our web
application. I did this in a .NET 4.0 class library project and wrote
integration tests to ensure the API integrated with the backend service
correctly. In the integration tests (a unit test project), as well as a
console application, the API's work correctly and return all the expected
results. However, when we execute the same API's from a ASP.NET web page
that is running under IIS, the API fails at the following line of code:
using (HttpWebResponse response = (HttpWebRequest)request.GetResponse())
The failure is a WebException with a status of SendFailure and a socket
error of ConnectionReset (10054) in the inner exception. The error is The
underlying connection was closed: An unexpected error occurred on a send.
This is using HTTPS, as well (hence the X509).
I already know that this is actually when the request is made, but I'm
trying to pin-point what is different about an IIS environment that would
prevent the stream from being able to write bytes over the network. I know
that this is actually the web service server closing the connection before
we get a chance to send our data, but I want to urge, again, that this
same API works fine under a integration or unit test, or console
application all day long.
I have already exhausted as many articles and posts on the internet that
are related that I could find, including extensive Msdn documentation such
as checking things related to the certificate, modifying HTTP headers
service point properties. I'm truly at a loss because the code is not
complicated, I've written web request code too many times to count, but
here it is:
private string ExecuteServiceMessages(Uri serviceUrl, X509Certificate
clientCertificate, string requestBody)
{
HttpWebRequest request =
(HttpWebRequest)HttpWebRequest.Create(serviceUrl);
request.ClientCertificates.Add(clientCertificate);
request.Date = DateTime.Now;
request.Method = WebRequestMethods.Http.Post;
request.ContentType = MediaTypeNames.Text.Xml;
request.UserAgent = "******";
request.KeepAlive = false;
using (Stream requestStream = request.GetRequestStream())
using (StreamWriter writer = new StreamWriter(requestStream))
{
writer.Write(requestBody);
writer.Close();
}
using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
using (Stream responseStream = response.GetResponseStream())
using (StreamReader reader = new StreamReader(responseStream))
{
string data = reader.ReadToEnd();
reader.Close();
return data;
}
}
The certificate is being loaded in by X509Certificate.CreateFromCertFile,
where our certificate for testing is just in a directory of the website.
We're not pulling it directly from the certificate store (unless we don't
fully understand how certificates work when loaded from a file).