我最近也遇到了同样的问题。我的环境运行在。net 4.6.1和VB.NET下。这就是我解决问题的方法:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3
ServicePointManager.ServerCertificateValidationCallback = New RemoteCertificateValidationCallback(AddressOf util.ValidateServerCertificate)

还有util。ValidateServerCertificate函数为:

Public Function ValidateServerCertificate(ByVal sender As Object, ByVal certificate As X509Certificate, ByVal chain As X509Chain, ByVal sslPolicyErrors As SslPolicyErrors) As Boolean
    Return True
End Function

在我的情况下，这个异常的根源是在代码的某些时候，下面的代码被调用:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;

这真的很糟糕。它不仅指示. net使用不安全的协议，而且还会影响在您的应用域中随后发出的每一个新的WebClient(和类似的)请求。(请注意，传入的web请求在你的ASP中不受影响。NET应用程序，但新的WebClient请求，如与外部web服务对话，是)。

在我的情况下，它实际上并不需要，所以我可以删除该语句，所有其他的web请求重新开始正常工作。根据我在其他地方的阅读，我了解到一些事情:

This is a global setting in your appdomain, and if you have concurrent activity, you can't reliably set it to one value, do your action, and then set it back. Another action may take place during that small window and be impacted. The correct setting is to leave it default. This allows .NET to continue to use whatever is the most secure default value as time goes on and you upgrade frameworks. Setting it to TLS12 (which is the most secure as of this writing) will work now but in 5 years may start causing mysterious problems. If you really need to set a value, you should consider doing it in a separate specialized application or appdomain and find a way to talk between it and your main pool. Because it's a single global value, trying to manage it within a busy app pool will only lead to trouble. This answer: https://stackoverflow.com/a/26754917/7656 provides a possible solution by way of a custom proxy. (Note I have not personally implemented it.)

这样做帮助了我:

System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

您可以尝试安装演示证书(一些ssl提供程序提供一个月的免费演示证书)，以确定问题是否与证书有效性有关。

这一个是为我工作在MVC webclient

public string DownloadSite(string RefinedLink)
{
    try
    {
        Uri address = new Uri(RefinedLink);

        ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;

        System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

        using (WebClient webClient = new WebClient())
        {
            var stream = webClient.OpenRead(address);
            using (StreamReader sr = new StreamReader(stream))
            {
                var page = sr.ReadToEnd();

                return page;
            }
        }

    }
    catch (Exception e)
    {
        log.Error("DownloadSite - error Lin = " + RefinedLink, e);
        return null;
    }
}

另一种可能是箱子上的证书输入不当。请确保选中周围的复选框。最初我没有这样做，所以代码要么超时，要么抛出相同的异常，因为私钥无法定位。