Error 500 InternalServerError when use httprequest call API (python flask) with long time response

Question

I build a service to call API from Python flask. This is my Service

public string CheckStatus(string json)
    {
        Utils log = new Utils();
        string ResponseString = "";
        HttpWebResponse response = null;
        try
        {
            string schedulingAPIUrl = ConfigurationManager.AppSettings["SchedulingAPI_URL"];
            var request = (HttpWebRequest)WebRequest.Create(string.Format(schedulingAPIUrl + "{0}", "CheckStatus"));
            request.Accept = "application/json";
            request.Method = "POST";
            request.Timeout = Timeout.Infinite;
            request.KeepAlive = true;
            var myContent = json;
            
            var data = Encoding.ASCII.GetBytes(myContent);

            request.ContentType = "application/json";
            request.ContentLength = data.Length;

            using (var stream = request.GetRequestStream())
            {
                stream.Write(data, 0, data.Length);
            }

            response = (HttpWebResponse)request.GetResponse();
            ResponseString = new StreamReader(response.GetResponseStream()).ReadToEnd();
        }
        catch (WebException ex)
        {
            if (ex.Status == WebExceptionStatus.ProtocolError)
            {
                response = (HttpWebResponse)ex.Response;
                ResponseString = "Some error occurred: " + response.StatusCode.ToString();
            }
            else
            {
                ResponseString = "Some error occurred: " + ex.Status.ToString();
            }
        }
        return ResponseString;
    }

My Service will transfer a parameter is a list of many dictionaries to my API. If the Parameter has an amount of ~ 10000 dictionaries, everything is ok. But If it has to amount to 1 million dictionaries or more, after my API run 8-10minutes, I get an error: "500 InternalServerError".

I use wfastcgi, IIS to deploy my API.

I don't know the reason for the 500 error being from my Service or my API.