While HttpClient gives bad gateway error, Postman works well for same rest endpoint

Question

I have a problem about 502 gateway error in my production server. I try to develop an application which calls azure rest endpoints. I get response with Postman by sending the endpoint which has following documentation:

https://docs.microsoft.com/en-us/rest/api/azure/devops/wit/classification-nodes/get?view=azure-devops-rest-6.0

While Postman or Invoke-RestMethod work well with azure endpoint, but API, developed by me, based on HttpClient/RestSharp give an 502 Bad gateway error. HttpClient code part can be found below:

var personalaccesstoken = "<my-token>";

using (HttpClient client = new HttpClient())
{
    client.DefaultRequestHeaders.Accept.Add(
        new MediaTypeWithQualityHeaderValue("application/json"));

    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic",
        Convert.ToBase64String(ASCIIEncoding.ASCII.GetBytes(string.Format("{0}:{1}", "<my-email-address>", personalaccesstoken))));

    HttpRequestMessage requestMessage = new HttpRequestMessage();
    requestMessage.Method = HttpMethod.Get;
    requestMessage.RequestUri = new Uri("<my-azure-server-address>/_apis/wit/classificationNodes/Areas?$depth=100");
    using (HttpResponseMessage response = client.SendAsync(requestMessage).Result)
    {
        response.EnsureSuccessStatusCode();
        string responseBody = await response.Content.ReadAsStringAsync();
        Console.WriteLine(responseBody);
        return responseBody;
    }
}

How can I solve the problem or which tool can I use to find the exact problem?

Thank you in advance: