Why does the .modal-backdrop overlap the modal window, the z-index seems to be like the modal window has more?

Question

HTML

Hello, I inserted the usual regular modal window from the manual for the test:

<!-- Button trigger modal -->
<button class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
        Посмотреть демо
      </button>

<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
        <h4 class="modal-title" id="myModalLabel">Название модали</h4>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Закрыть</button>
        <button type="button" class="btn btn-primary">Сохранить изменения</button>
      </div>
    </div>
  </div>
</div>

When you call it, the .modal-backdrop dimming appears and it appears on top of the modal window..respectively, the modal window is not active.
The z-index of the modal window is higher in the bootstrap, respectively.
What could be the problem?
Everything works fine in the manual.