Strange object exception thrown in HttpWebResponse

I am working on a program that automatically requests a website every 5 seconds. It has been working fine for the past few days, but today, when I just restarted it, it continues to throw System.ObjectDisposedExceptionon the line marked below. I should mention that accessing this URL through a browser on the same computer works fine.

The code:

HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://www.bitstamp.net/api/ticker/");

request.Method = "GET";

try
{
    // ObjectDisposedException thrown here
    using (HttpWebResponse response = (HttpWebResponse)request.GetResponse()) 
    {
        using (StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.UTF8))
        {
            string resultString = reader.ReadToEnd();
            return resultString;
        }
    }
}
catch (WebException ex)
{
    // Handle it
}

Stack trace:

System.ObjectDisposedException occurred
    _HResult=-2146232798
   _message=Cannot access a disposed object.
    HResult=-2146232798
    IsTransient=false
    Message=Cannot access a disposed object.
Object name: 'SslStream'.
Source=System
    ObjectName=SslStream
StackTrace:
at System.Net.Security.SslState.ValidateCreateContext(Boolean isServer, String targetHost, SslProtocols enabledSslProtocols, X509Certificate serverCertificate, X509CertificateCollection clientCertificates, Boolean remoteCertRequired, Boolean checkCertRevocationStatus, Boolean checkCertName)
 InnerException: 

Is there something I'm doing wrong? I don’t even get access to the response stream until usinghow can I delete it?

EDIT: Added URL tracing and tracing