<!-- Button Modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModalButton">
  Klick mich
</button>
<!-- Modal -->
<div class="modal fade" id="exampleModalButton" tabindex="-1" role="dialog" aria-labelledby="exampleModalLongTitle" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <h5 class="modal-title" id="exampleModalLongTitle">Modal Titel</h5>
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="false">×</span>
        </button>
      </div>
      <div class="modal-body">
        <p>Das ist der Body von diesem <b>Modal</b></p><p>
      </p></div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Schließen</button>
        <!-- <button type="button" class="btn btn-primary">Button</button> -->
      </div>
    </div>
  </div>
</div>
Bemerkung: seit Mobirise v4.9.7 musste ich die Höhe vom Inhalt eines jeden Modals mit einer CSS begrenzen, wie z.B.:
.modal-content {
  max-height: 300px;
  overflow-y: auto;
}