I realize the post is old... but not finding any real good solutions yet...

my domain jeffersonvaughn.com is hosting a website in which I am running a .net core 3.1 application that executes a POST method HTTP request to server jvaughn1.powerbunker.com

I have generated a certificate on the jvaughn1.powerbunker.com server and have exported to a .pfx file in which I'm using in my .net application.

this http api request along with this .pfx file works when using Postman.

When my .net application makes the request I get...

An unhandled exception occurred while processing the request.
AuthenticationException: The remote certificate is invalid according to the validation procedure.
System.Net.Security.SslStream.StartSendAuthResetSignal(ProtocolToken message, AsyncProtocolRequest asyncRequest, ExceptionDispatchInfo exception)

HttpRequestException: The SSL connection could not be established, see inner exception.
System.Net.Http.ConnectHelper.EstablishSslConnectionAsyncCore(Stream stream, SslClientAuthenticationOptions sslOptions, CancellationToken cancellationToken)

my .net code looks like this...

startup...

            var cert = new X509Certificate2(@"C:\Users\jeffe\Desktop\Corei Solutions\products\core-i_rst\Apache HTTP_SSL\jvaughn1.powerbunker.com.pfx"
                                           , "corei");

            var handler = new HttpClientHandler()
            {
                SslProtocols = SslProtocols.Tls13 | SslProtocols.Tls12 | SslProtocols.Tls11 | SslProtocols.Tls
            };

            handler.ClientCertificates.Add(cert);
            services.AddHttpClient("coreiClient", c =>
            {
            }).ConfigurePrimaryHttpMessageHandler(() => handler);

            // handles issue with non-trusted certs...
            ServicePointManager.ServerCertificateValidationCallback += delegate { return true; };

controller...

        var request = new HttpRequestMessage
        {
            Method = HttpMethod.Post,
            //RequestUri = new Uri(Url + jsonRequest),
            RequestUri = new Uri(g_Url),
            Content = new StringContent(jsonRequest, System.Text.Encoding.Default, "text/plain"),
        };

        // for certificate authentication...
        var client = _clientFactory.CreateClient("coreiClient");

        client.DefaultRequestHeaders.Accept.Add(
            new MediaTypeWithQualityHeaderValue("application/json"));

Is there ANY possible way you can help shed some light on this and save me from this nightmare I been trying to wake up from!!!??? 😃