HttpClient object does not works second time on Xamarin.iOS

Question

I use HttpClient object for PostAsync. I need to add BackgroundSessionConfiguration for iOS while I am creating HttpClient object. So I changed my code like this:

var configuration = NSUrlSessionConfiguration.CreateBackgroundSessionConfiguration ("my.app.identifier");
_client = new HttpClient (new NSUrlSessionHandler (configuration));

This works when I send first request with PostAsync. But when I send request second time, it doesn't work.

I did it for Login Operation like this: (It works first time but if I logout and login again, it doesn't work.)

public class LoginService
    {
        private HttpClient _client;

        public LoginService()
        {
            if (_client == null)
                {
                    _client = Helper.CreateHttpClientLogin(_client);
                }
        }

    public async Task<LoginResponse<LoginDataResponse>> Login(LoginRequest request)
        {
            LoginResponse<LoginDataResponse> responseModel = new LoginResponse<LoginDataResponse>();
            try
            {
                string json = JsonConvert.SerializeObject(request);
                var content = new StringContent(json, Encoding.UTF8, "application/json");

                var jsonBody = await _client.PostAsync(App.ServiceURL.Login_Url, content);
                string jsonstr = await jsonBody.Content.ReadAsStringAsync();

                if (jsonstr == null || jsonstr == "")
                {
                    responseModel.Success = false;
                    responseModel.Status = 0;
                    responseModel.Message = AppResources.UnknownHostException;
                }
                else
                    responseModel = (LoginResponse<LoginDataResponse>)JsonConvert.DeserializeObject(jsonstr, typeof(LoginResponse<LoginDataResponse>));
            }
            catch (Exception ex)
            {
                string text = ex.ToString();
                responseModel.Status = 0;
                AppResources.Culture = CrossMultilingual.Current.CurrentCultureInfo;
                responseModel.Message = AppResources.UnknownHostException;
            }
            return responseModel;
        }
}

public class Helper
{
    public static HttpClient CreateHttpClientLogin(HttpClient _client)
    {
            if (Device.RuntimePlatform == Device.iOS)
                {
                    var configuration = NSUrlSessionConfiguration.CreateBackgroundSessionConfiguration("my.app.identifier");
                    _client = new HttpClient(new NSUrlSessionHandler(configuration));
                }
                else
                {
                    //_client = new HttpClient(new System.Net.Http.HttpClientHandler());
                    HttpClientHandler handler = new HttpClientHandler();
                    handler.ServerCertificateCustomValidationCallback = (message, cert, chain, errors) => true;
                    _client = new HttpClient(handler);
                }

            return _client;
        }
}

And I have this code on AppDelegate: (I don't know but maybe it causes the bug)

public static Action BackgroundSessionCompletionHandler;
public override void HandleEventsForBackgroundUrl(UIApplication application, string sessionIdentifier, Action completionHandler)
{
    // We get a completion handler which we are supposed to call if our transfer is done.
    BackgroundSessionCompletionHandler = completionHandler;
}

What must I do for this?

Edit:

I solved the problem I mentioned above by creating the Login Service object once the application was first opened. (After logout previously, I was rebuilding every time I login)

But now I have other error. When I run my app on "iPhone 7 plus - iOS 13.6" device I got this error:

System.Net.Http.HttpRequestException: unknown error ---> Foundation.NSErrorException: Error Domain=NSURLErrorDomain Code=-1 "unknown error" UserInfo={NSErrorFailingURLStringKey=https://mydomain/Api/Login, NSErrorFailingURLKey=https://mydomain/Api/Login, _NSURLErrorRelatedURLSessionTaskErrorKey=(
    "BackgroundDataTask <E69F3EAF-0AE9-4FAE-A01B-988167B7F6BC>.<3>"
), _NSURLErrorFailingURLSessionTaskErrorKey=BackgroundDataTask <E69F3EAF-0AE9-4FAE-A01B-988167B7F6BC>.<3>, NSLocalizedDescription=unknown error}
   --- End of inner exception stack trace ---
  at System.Net.Http.NSUrlSessionHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) [0x001d4] in /Library/Frameworks/Xamarin.iOS.framework/Versions/13.20.2.2/src/Xamarin.iOS/Foundation/NSUrlSessionHandler.cs:527 
  at System.Net.Http.HttpClient.FinishSendAsyncBuffered (System.Threading.Tasks.Task`1[TResult] sendTask, System.Net.Http.HttpRequestMessage request, System.Threading.CancellationTokenSource cts, System.Boolean disposeCts) [0x0017e] in /Library/Frameworks/Xamarin.iOS.framework/Versions/Current/src/Xamarin.iOS/external/corefx/src/System.Net.Http/src/System/Net/Http/HttpClient.cs:506 
  at App.Services.LoginService.Login (FileOrbis.Models.RequestModels.LoginRequest request) [0x00084] in C:\Users\PcName\Desktop\App\App\Services\LoginService.cs:40

And simulator log file is:

Startup:
    arguments: --device=06098E5B-1853-4A83-8434-8071D8973A14 --launchsim=//Users/deytek/Library/Caches/Xamarin/mtbs/builds/App.iOS/b2c75f2acbd4ff91c305dba10ca791b7/bin/iPhoneSimulator/Debug/App.iOS.app -argument=-monodevelop-port -argument=51890 -setenv=__XAMARIN_DEBUG_PORT__=51890 --sdkroot=/Applications/Xcode.app -h=192.168.1.7 -ssh=deytek --launched-by=devenv-16.0 
    version: 16.7.0.0 (54a29526ef6f853bdd37adbcc3791ce90ca82735)
Connecting to existing client
Exit:
    Exit Code: 0

I encounter with this error when I use Background Session Configuration. If I use normal HttpClient object (without Background Session Configuration), it works

NOTE: I also tried iPhone 5s iOS 12.4.8 and iPad Pro (3rd Generation) iOS 13.6.1 It works these devices. But it doesn't work on iPhone 7 Plus 13.6