If you are serializing your data object, it will not be a proper json object. Take what you have, and just wrap the data object in a JSON.stringify()
.
$http({
url: '/user_to_itsr',
method: "POST",
data: JSON.stringify({application:app, from:d1, to:d2}),
headers: {'Content-Type': 'application/json'}
}).success(function (data, status, headers, config) {
$scope.users = data.users; // assign $scope.persons here as promise is resolved here
}).error(function (data, status, headers, config) {
$scope.status = status + ' ' + headers;
});