controlla bene i percorsi, che siano tutti su https, e metti un autocomplete="off" per l'input:

codice:
<script src="https://code.jquery.com/jquery-3.5.1.js" integrity="sha256-QWo7LDvxbWT2tbbQ97B53yJnYU3WhH/C8ycbRAkjPDc=" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/jquery-validation@1.17.0/dist/jquery.validate.min.js"> </script>


<script src="https://code.jquery.com/ui/1.9.1/jquery-ui.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.11.4/i18n/jquery-ui-i18n.min.js">
</script>


<link rel="stylesheet" href="https://code.jquery.com/ui/1.9.2/themes/base/jquery-ui.css" />


  <script>
  $( function() {   
    $.datepicker.setDefaults($.datepicker.regional['it']);
    $("#datepicker").datepicker() ;
    $("#datepicker2").datepicker() ;
  } );
  </script>
codice:
<input type="text" id="datepicker" autocomplete="off" name="data" maxlength="150" value="">
<input type="text" id="datepicker2" autocomplete="off" name="data" maxlength="150" value="">