System.Net.WebException：错误：TrustFailure（对SSPI的调用失败，请参阅内部异常。）

Question

xamarin中出现以下错误：System.Net.WebException：错误：TrustFailure（对SSPI的调用失败，请参阅内部异常。）

下面是我的代码：

            UserRepository usr = new UserRepository();
            var restClient = usr.RestClient;         //rest client code
            restClient.BaseUrl = StdAppIds.webServerUrl;
            var restRequest = new RestRequest();
            restRequest.Resource = "/GetVersion";
            restRequest.Method = HttpMethod.POST;
            var response = await restClient.ExecutePostAsync(restRequest);  //rest request