WebException/TlsException during HTTPS Request

I'm trying to communicate with my REST API through HttpWebRequest/Response. I managed to import the CAs from mozroots.exe, but I'm still getting WebExceptions/TlsExceptions.

Here is some example code that tries to use Flickr API test:

 using System.IO;
 using System.Net;
 using UnityEngine;
 
 public class TestScript : MonoBehaviour
 {
     void Start()
     {
         HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://api.flickr.com/services/rest/?method=flickr.test.echo&name=12345");
         request.Method = "POST";
         HttpWebResponse response = (HttpWebResponse)request.GetResponse();
         StreamReader reader = new StreamReader(response.GetResponseStream());
         Debug.Log(string.Format("Code: {0}\nDesc: {1}\nType: {2}\nLength: {3}\nEncoding: {4}\nContent: {5}", response.StatusCode, response.StatusDescription, response.ContentType, response.ContentLength, response.ContentEncoding, reader.ReadToEnd()));
         reader.Close();
         response.Close();
     }
 }
 

And here is the error:

 TlsException: Invalid certificate received from server. Error code: 0xffffffff80092012
 Mono.Security.Protocol.Tls.Handshake.Client.TlsServerCertificate.validateCertificates (Mono.Security.X509.X509CertificateCollection certificates)
 Mono.Security.Protocol.Tls.Handshake.Client.TlsServerCertificate.ProcessAsTls1 ()
 Mono.Security.Protocol.Tls.Handshake.HandshakeMessage.Process ()
 (wrapper remoting-invoke-with-check) Mono.Security.Protocol.Tls.Handshake.HandshakeMessage:Process ()
 Mono.Security.Protocol.Tls.ClientRecordProtocol.ProcessHandshakeMessage (Mono.Security.Protocol.Tls.TlsStream handMsg)
 Mono.Security.Protocol.Tls.RecordProtocol.InternalReceiveRecordCallback (IAsyncResult asyncResult)
 Rethrow as IOException: The authentication or decryption has failed.
 Mono.Security.Protocol.Tls.SslStreamBase.AsyncHandshakeCallback (IAsyncResult asyncResult)
 Rethrow as WebException: Error getting response stream (Write: The authentication or decryption has failed.): SendFailure
 System.Net.HttpWebRequest.EndGetResponse (IAsyncResult asyncResult)
 System.Net.HttpWebRequest.GetResponse ()
 TestScript.Start () (at Assets/TestScript.cs:11)
 

Does anyone know what I'm doing wrong?