Why is my API throws an error when using authentication mode

Question

Somehow when using Windows mode, I get an Exception Error"Reference to undeclared entity 'nbsp'. Line 69, position 41."" weird. Then when I check the result.Content after executing I get like an html page with errors like 401 - Unauthorized: Access is denied due to invalid credentials. This is an issue that happens on the IIS Server too.

Below is my RestSharp Code from ActionResult

var client = new RestClient(Request.Url.GetLeftPart(UriPartial.Authority).ToString());
            var request = new RestRequest("http://domain.co.za/api/student/getBookedSlotHistory", Method.POST);
               request.AddHeader("cache-control", "no-cache");
               request.AddObject(new StudentInput()
               {
                   StudentNumber = "219193029"//UserIdentity.Username()
               });

               var result = client.Execute(request);
               var data = JsonConvert.DeserializeObject(result.Content);

Then below is my APIController

[HttpPost]
       [Route("api/student/getBookedSlotHistory")]
       public async Task GetHistory(StudentInput input)
       {

           return await _studentRepository.GetBookingHistoryData(input);
       }