.ashx-远程服务器返回错误:(500)内部服务器错误

我在单个方法中一个接一个地对一个远程REST service进行两次调用。我在第一次调用中设置accessToken的值，并将其用于第二次请求。

当我运行它时，它会给我错误作为

The remote server returned an error: (500) Internal Server Error.

以下是代码。

    HttpWebRequest webRequest = null;
    HttpWebResponse webResponse = null;
    Encoding encodingObj = null;
    StreamReader streamReaderObj = null;
    string grantCode = string.Empty;
    string resultString = string.Empty;
    string accessToken = string.Empty;
    private void Instantiate()
    {            
        grantCode = HttpContext.Current.Request.QueryString["code"].ToString();
        webRequest = (HttpWebRequest)WebRequest.Create(Constants.ACCESS_TOKEN_REQUEST + "&code=" + grantCode);
        webRequest.Method = "GET";
        webRequest.ContentType = "application/json";
        webResponse = (HttpWebResponse)webRequest.GetResponse();
        encodingObj = System.Text.Encoding.GetEncoding("utf-8");
        streamReaderObj = new StreamReader(webResponse.GetResponseStream(), encodingObj);
        resultString = streamReaderObj.ReadToEnd();
        JObject parameterCollection = JObject.Parse(resultString);
        accessToken = parameterCollection["access_token"].ToString();
        //HttpContext.Current.Response.Write("<br/><br/>Code: <br/>" + grantCode);
        //HttpContext.Current.Response.Write("<br/><br/>Access Token: <br/>" + accessToken);
        webRequest = (HttpWebRequest)WebRequest.Create(Constants.RETRIEVE_CONTEXT_REQUEST + "vista-688/id/Staff01");
        webRequest.Method = "GET";
        webRequest.Accept = "application/json";
        webRequest.ContentType = "application/json";
        webRequest.Headers.Add("Authorization", "Bearer " + accessToken);
        webResponse = (HttpWebResponse)webRequest.GetResponse();
        encodingObj = System.Text.Encoding.GetEncoding("utf-8");
        streamReaderObj = new StreamReader(webResponse.GetResponseStream(), encodingObj);
        resultString = streamReaderObj.ReadToEnd();
        //HttpContext.Current.Response.Write("<br/><br/>Retrieve Context: <br/>" + resultString);
    }

这些是来自配置文件的完整rest api URL：

<add key="GrantCodeRequest" value="https://<location>/AuthorizationServices/provider/authorize?response_type=code&state=mystateid&client_id=mVisum&redirect_uri=http://localhost:1316/RetrieveContext.aspx&scope=read"/>
<add key="AccessTokenRequest" value="https://<location>/AuthorizationServices/oauth/token?client_id=mVisum&state=mystateid&scope=read&client_secret=TESTMVISUM&response_type=token&grant_type=authorization_code&redirect_uri=http://localhost:1316/RetrieveContext.aspx"/>
<add key="RetrieveContextRequest" value="http://<location>/UserContext/rest/context/user/system/"/>

当我只执行accessToken值初始化为有效值的第二个请求时，第二个调用也在正常工作。这个方法是在一个处理程序中编写的。

有人能告诉我为什么会发生这种事吗？REST web服务中没有问题。我也尝试过使用两个独立的web请求和web响应对象，但没有任何结果

        WebClient client = new WebClient();
        client.Headers["Content-type"] = @"application/json";
        Stream data = client.OpenRead(yoururl); ;
        StreamReader reader = new StreamReader(data);
        string responseFromServer = reader.ReadToEnd();