ASP.NET Web API 2 - The request was aborted: Could not create SSL/TLS secure channel

Question

Every other answer to "The request was aborted: Could not create SSL/TLS secure channel" essentially says "make sure TLS 1.2 is on". This has not worked, and I have the Wireshark captures to back it up.

Now, I have a .NET Framework 4.7.2 ASP.NET Web API running in IIS. I am trying to interact with a 3rd-party API that requires a client certificate for mTLS. When my api sends the initial login request, I get the error The request was aborted: Could not create SSL/TLS secure channel. The exception does not seem to contain any details about cause and does not have an inner exception.

The crazy part is, I only get this error when my API is running in IIS on a Windows 2022 server (AWS EC2). I do not get the error when I test locally. Furthermore, I do not get the error when I copy the exact code from my API into a console app and run that on the same server that the API runs on.

The certificate is loaded like so:

using (var s3Client = new AmazonS3Client())
{
    var response = s3Client.GetObject(bucket, key);

    using (response.ResponseStream)
    using (var file = new MemoryStream())
    {
        response.ResponseStream.CopyTo(file);
        cert = new X509Certificate2(file.ToArray(), certPassword);
    }
}

and used like so:

var httpClientHandler = new HttpClientHandler();
httpClientHandler.ClientCertificates.Add(cert);

_httpClient = new HttpClient(httpClientHandler)
{
    BaseAddress = _config.BaseUri
};

In Wireshark, it seems like the conversation just cuts off.

Here is what success looks like (console app): Success

Here is what failure looks like (ASP.NET Web API 2) Failure

From answers to similar questions, I have tried:

• adding every registry entry related to enabling TLSv1.2 on Windows.
• adding ServicePointManager.SecurityProtocol = SecurityProtocolType.tls12 to the code.
• adding ServicePointManager.ServerCertificateValidationCallback += (sender, certificate, chain, errors) => true to the code.
• making sure that httpRuntime is set to 4.7.2 in the Web.config.

Nothing made any difference and Wireshark shows TLSv1.2 is happening.