C# – How to trace the HttpClient request using fiddler or any other tool

What's the client of the web service? ASP.NET?

ASP.NET traffic isn't proxied unless you configure ASP.NET to use a proxy. It's possible/likely that the app.config or machine.config changed such that traffic is no longer getting proxied?

You should have a look at this section: http://www.fiddlerbook.com/fiddler/help/hookup.asp#Q-DOTNET

The content type is a header of the content, not of the request, which is why this is failing. AddWithoutValidation as suggested by Robert Levy may work, but you can also set the content type when creating the request content itself (note that the code snippet adds application/json in two places-for Accept and Content-Type headers):

HttpClient client = new HttpClient();
client.BaseAddress = new Uri("http://example.com/");
client.DefaultRequestHeaders
      .Accept
      .Add(new MediaTypeWithQualityHeaderValue("application/json"));//ACCEPT header

HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, "relativeAddress");
request.Content = new StringContent("{\"name\":\"John Doe\",\"age\":33}",
                                    Encoding.UTF8, 
                                    "application/json");//CONTENT-TYPE header

client.SendAsync(request)
      .ContinueWith(responseTask =>
      {
          Console.WriteLine("Response: {0}", responseTask.Result);
      });