responseUri C# problem

I am trying to set my dropbox configuration to TokenAccessType Offline on my page, but I am getting erros on the "DropboxOAuth2Helper.ProcessCodeFlowAsync"

I get: Dropbox.Api.OAuth2Exception: 'invalid_grant'

for error description I got: redirect_uri mismatch

after I got the page to enter my credentials I call this method:

 

public async Task<ActionResult> DropboxRedirect(string code, string state)
        {
            try
            {
                var connectState = PantherCache.Get(_identityService.User.Guid + "_dropBoxConnectionState").Replace("\"", "");


                if (connectState != state)
                {
                    SessionVars.AddNotification(User.Identity.Name, new Notification
                    {
                        Type = NotificationType.error,
                        Text = "There was an error connecting to Dropbox."
                    });
                    return this.RedirectToAction("BoxSync");
                }

                var appKey = ConfigurationManager.AppSettings["dropbox.clientId"].ToString();
                var appSecret = ConfigurationManager.AppSettings["dropbox.clientSecret"].ToString();
                string redirectUri = Url.Action("DropboxRedirect", "Settings", new { code = code, state = state }, this.Request.Url.Scheme);
                Uri responseUri = new Uri(redirectUri);
                var response = await  DropboxOAuth2Helper.ProcessCodeFlowAsync(responseUri, appKey, appSecret, redirectUri.ToString(), state: connectState);
               // OAuth2Response response = await DropboxOAuth2Helper.ProcessCodeFlowAsync(responseUri, appKey, appSecret, state: connectState);

                var tenant = await _unitOfWork.TenantRepository.GetTenantByTenantGuid(_identityService.Tenant.Guid).FirstAsync();
                var isDropboxIntegrationEnabled = tenant.IsDropboxIntegrationEnabled;
                tenant.DropboxAuthToken = Encryptor.Encrypt(response.AccessToken);
                tenant.IsDropboxIntegrationEnabled = true;
                var res = await _unitOfWork.TenantRepository.InsertOrUpdateAndSaveAsync(tenant);

                //if the tenant was not integrated to drop box prior to the update
                if (!isDropboxIntegrationEnabled)
                {
                    QueueService.SendDropboxSyncQueueMessage(tenant.Id, null, null, Model.QueueModels.BoxSyncMessageType.InitSync, _identityService.User.Guid);
                }

                _identityService.ResetSessionVariablesFromCurrentContext(_identityService.User.Name, true, true);



                SessionVars.AddNotification(User.Identity.Name, new Notification
                {
                    Type = NotificationType.success,
                    Text = "This account is now connected to Dropbox!"
                });
                return this.RedirectToAction("BoxSync");
            }
            catch (Exception e)
            {
                SessionVars.AddNotification(User.Identity.Name, new Notification
                {
                    Type = NotificationType.error,
                    Text = "DropBox Authentication Error. Please try again later or contact support."
                });
                ErrorHelper.LogErrorManually(e);
                return this.RedirectToAction("BoxSync");
            }

        }

 

 

I got the error on this line:
var response = await DropboxOAuth2Helper.ProcessCodeFlowAsync(responseUri, appKey, appSecret, redirectUri.ToString(), state: connectState);

currently my ResponseUri values are:  "https://localhost:44383/Settings/DropboxRedirect?code=qX6NI6vbQh4AAAAAAAAAbCeVf-D8ZK916ZSKjp7oc64&state=afeb873a8cb7437dac1da70a59690dab"

I am creating my responseUri wrong? how should I create it?