I changed your code to this

                            if (response.IsSuccessStatusCode)
                            {
                                var result = await response.Content.ReadAsStringAsync();
                                // result shows a valid string. 
                                var promotemp = JsonConvert.DeserializeObject<List<DetailsHeader>>(result);
                             }

And if failed with the same error as my code did. Has to be a problem with the DetailsHeader class. What is really weird is I have another class that is pulling different data including a DateTime and it is working fine. I think I am going to pull this into a temp project without Xamarin and see what happens.