Dear Claudio Biferale,
Your site miss modal.js file. Please go to your_site\templates\sj_jare25\includes\head.php and find:
<!-- Include js -->
<script src="<?php echo $yt->templateurl().'js/yt-script.js' ?>" type="text/javascript"></script>
<script src="<?php echo $yt->templateurl().'js/prettify.js' ?>" type="text/javascript"></script>
=>
<!-- Include js -->
<script src="<?php echo $yt->templateurl().'js/yt-script.js' ?>" type="text/javascript"></script>
<script src="<?php echo $yt->templateurl().'js/prettify.js' ?>" type="text/javascript"></script>
<?php JHTML::_('behavior.modal'); ?>
Thanks
