基础连接已经关闭: 未能为 SSL/TLS 安全通道建立信任关系。

InnerException异常类型：AuthenticationException
异常消息：根据验证过程，远程证书无效。

解决方案：操作系统的当前时间不对。

------------------

The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

INNER: The remote certificate is invalid according to the validation procedure.

解决方案：ROOT CA 证书未安装。（假设你的证书厂商是 digicert，就要安装 digicert ROOT CA 证书到操作系统-受信息的根证书颁发机构）

------------------

2019-8-29，更新：

环境：WIN SERVER 2003 遇到 The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel. ++ InnerException异常类型：AuthenticationException 。

解决：

1 安装 WindowsServer2003-KB938397-x86-CHS.exe 补丁；

2 IE- Internet 选项- 高级 - 勾选 TLS1.0。

------------------