基础连接已经关闭: 未能为 SSL/TLS 安全通道建立信任关系

//1.
ServicePointManager.ServerCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) => true;

//2.
ServicePointManager.ServerCertificateValidationCallback = new        
RemoteCertificateValidationCallback
(
   delegate { return true; }
);

请求被终止: 未能为 SSL/TLS 安全通道建立信任关系

注释 ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3; 后恢复正常