PayPal取消订阅（.Net Core）时发生500 INTERNAL_SERVICE_ERROR

Question

实施贝宝订阅：

1. 通过PayPal付款标准进行订阅。
2. 通过PayPal API取消订阅（不带SDK）。

我在通过PayPal API取消订阅/定期付款时遇到问题。我能够通过API获得AccessToken。以下是我取消订阅的代码：

string url = "https://api.sandbox.paypal.com/v1/payments/billing-agreements/" + mySubscriberId + "/cancel";
HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
webRequest.Method = "POST";
webRequest.ContentType = "application/x-www-form-urlencoded";
webRequest.Accept = "application/json";
webRequest.Headers.Add("Authorization: Bearer " + myPaypalAccessToken);
var request = ("note=" + myCancelNote);
byte[] req_bytes = Encoding.ASCII.GetBytes(request);
webRequest.ContentLength = req_bytes.Length;

Stream strm = webRequest.GetRequestStream();
strm.Write(req_bytes, 0, req_bytes.Length);
strm.Close();

HttpWebResponse resp = (HttpWebResponse)webRequest.GetResponse();
using (Stream respStr = resp.GetResponseStream())
{
    using (StreamReader reader = new StreamReader(respStr, Encoding.UTF8))
    {
        string jsonString = await reader.ReadToEndAsync();
        reader.Close();
    }
 }

我每次都会收到以下答复：

“名称”：“ INTERNAL_SERVICE_ERROR”， “ message”：“发生内部服务错误。”， “ information_link”：“ https://developer.paypal.com/webapps/developer/docs/api/#INTERNAL_SERVICE_ERROR”， “ debug_id”：“ c121fd139b91b”

有人可以向我保证，如果我走的路正确，可能是问题的原因。谢谢！