Debugging failing HTTPS WebRequest

I'm writing a small program which will make a GET request to a server using HTTPS and the HttpWebRequest class. The server (obviously) has a server certificate. It also expects the client to provide a certificate.

When making the request, however, I get a System.Net.WebException stating that it wasn't possible to establish a secure TLS/SSL connection. I quickly discovered that the server's certificate wasn't valid. Assuming this was what was causing the exception, I tried to accept the invalid certificate (updating the certificate is, unfortunately, not an option) using the code below:

ServicePointManager.ServerCertificateValidationCallback += delegate {
    return true;
};

That didn't solve the problem, however.

Since the exception doesn't give any detail, it's hard to actually determine what is causing it. Is my attempt to override the invalid server certificate not working? Is the client certificate I'm providing not trusted by the server? Am I not loading the client certificate in the proper manner?

I'd love tips on how to debug this sort of problem. I do not have access to the server or its logs, unfortunately.

Below is the important parts of the code:

ServicePointManager.ServerCertificateValidationCallback += delegate {
    return true;
};
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url); // url is an HTTPS URL.
X509Certificate clientCert = new X509Certificate("certificate.crt", "password");
req.ClientCertificates.Add(clientCert);
WebResponse resp = req.GetResponse(); // This fails!