.NET 4.8 RestClient带证书、基础认证和代理调用报0X8009030D错误求助

我正在使用C# .NET 4.8应用调用一个通过证书和基础认证保护的Web服务，调用时出现错误：

System.Net Error: 0 : [3564] AcquireCredentialsHandle() failed with error 0X8009030D.

进而抛出：

The request was aborted: Could not create SSL/TLS secure channel.

我查阅了多篇帖子但未取得进展，相关代码如下：

ServicePointManager.SecurityProtocol = spt;
ServicePointManager.Expect100Continue = true;
ServicePointManager.ServerCertificateValidationCallback += (sender, cert, chain, sslPolicyErrors) => true;

RestClient client = new RestClient(uri);

client.Authenticator = new HttpBasicAuthenticator(username, password);

client.ClientCertificates = new X509CertificateCollection();
client.ClientCertificates.Add(new X509Certificate(certificate, certificatePassword));

client.Proxy = new WebProxy(proxy, port);
client.Proxy.Credentials = CredentialCache.DefaultCredentials;

request = new RestRequest(Method.POST);

request.AddHeader("Message-Key", messageKey);


request.RequestFormat = DataFormat.Json;
request.AddParameter("application/json", json, ParameterType.RequestBody);

IRestResponse response = client.Execute(request);

证书为pfx文件，使用SoapUI执行相同调用可成功。我已配置生成System.Net跟踪日志，日志中首先出现：

System.Net Error: 0 : [3564] Can't retrieve proxy settings for Uri 'https://dev.api.sydney.edu.au/usyd-message-handler-exp-api-v1/v1/messages'. Error code: 12180.

我认为此问题不大，因为未配置代理时调用会超时。日志末尾显示：

[Thumbprint]   0C1EFA37718255EE6DEF3A8A980CD30939762EA6 . System.Net Information: 0 : [3564] SecureChannel#14353717 - Found the certificate in the LocalMachine store. System.Net Information: 0 : [3564] SecureChannel#14353717::.AcquireClientCredentials, new SecureCredential() (flags=(ValidateManual, NoDefaultCred, SendAuxRecord, UseStrongCrypto), m_ProtocolFlags=(Ssl3Client, Tls12Client), m_EncryptionPolicy=RequireEncryption) System.Net Information: 0 : [3564] AcquireCredentialsHandle(package = Microsoft Unified Security Protocol Provider, intent  = Outbound, scc     = System.Net.SecureCredential) System.Net Error: 0 : [3564] AcquireCredentialsHandle() failed with error 0X8009030D. System.Net Information: 0 : [3564] AcquireCredentialsHandle(package = Microsoft Unified Security Protocol Provider, intent  = Outbound, scc     = System.Net.SecureCredential) System.Net Error: 0 : [3564] AcquireCredentialsHandle() failed with error 0X8009030D. System.Net.Sockets Verbose: 0 : [3564] Entering Socket#32115247::Dispose() System.Net Error: 0 : [3564] Exception in HttpWebRequest#24230272:: - The request was aborted: Could not create SSL/TLS secure channel.. System.Net Error: 0 : [3564] Exception in HttpWebRequest#24230272::EndGetRequestStream - The request was aborted: Could not create SSL/TLS secure channel.. System.Net Information: 0 : [10700] ServicePoint#16495015 - Closed as idle.

我尝试了ServicePointManager.SecurityProtocol的所有可能组合，若不设置或至少包含Tls12则出现此问题，否则会出现底层连接关闭错误。恳请提供解决思路。

内容的提问来源于stack exchange，提问作者Tony C