httpclient 502 error xamarin app trasnfering photo via json

Question

We are getting 502 error when we try to send photo in Json. App developed with .NET and Xamarin.Form

var jsonObjGuid = JsonConvert.SerializeObject(ObjGuid);
var jsonObjFiles = JsonConvert.SerializeObject(ObjFiles, Formatting.Indented);
var url = $"{ Session.EndpointURL}{MethodNames.UploadDossierFiles.Value}";

try
{
    using (HttpClient httpClient = new HttpClient())
    {
        httpClient.DefaultRequestHeaders.TransferEncodingChunked = true;
        httpClient.Timeout = TimeSpan.FromMilliseconds(600000);

        using (HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, url))
        {
            //request.Headers.Add(HeaderKeys.UserAgent.Value, Resources.DefaultUserAgent);
            request.Headers.Add(HeaderKeys.UserAgent.Value, "MobileApp");
            request.Headers.Add(HeaderKeys.Token.Value, token);

            HttpContent ObjGuidContent = new StringContent(jsonObjGuid);
            HttpContent ObjFilesContent = new StringContent(jsonObjFiles);

            MultipartFormDataContent content = new MultipartFormDataContent
                {
                    {ObjGuidContent, "ObjGuid"},
                    {ObjFilesContent, "ObjFiles"}
                };

            request.Content = content;
            var response = await Policy.HandleResult<HttpResponseMessage>(message => !message.IsSuccessStatusCode)
                .WaitAndRetryAsync(3, i => TimeSpan.FromSeconds(2), (result, timeSpan, retryCount, context) => { })
                .ExecuteAsync(() => httpClient.SendAsync(request));
        }
    }
}

Sometimes it works but often I get this error. Any help, suggestions? What do I need to check?