Linkedin API访问令牌获取异常

当我想要获得访问令牌时，它抛出WebException。下面的代码在6月21日之前获取访问令牌。在此之后，它抛出WebException

    var linkAccessToken = "https://www.linkedin.com/uas/oauth2/accessToken?grant_type=authorization_code&code=" + code + "&redirect_uri=" + redirectUri + "&client_id=" + clientId + "&client_secret=" + clientSecret;
    var request = (HttpWebRequest)WebRequest.Create(linkAccessToken);

    request.ContentType = "application/json; charset=utf-8";

    try
    {
        using (var response = request.GetResponse())
        {
        }
    }
    catch (WebException ex)
    {
        // ex.Status is WebExceptionStatus.SendFailure
    }

---

在创建请求之前，请使用以下代码

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;