Dear Alberto García,
The error relates to missing Mootools. Because our YT Plugin removed all Mootools. So, please go to the file: your_site\plugins\system\yt\yt.php and comment the code:
// Remove default mootools
unset($doc->_scripts[JURI::root(true) . '/media/system/js/core.js']);
unset($doc->_scripts[JURI::root(true) . '/media/system/js/modal.js']);
unset($doc->_scripts[JURI::root(true).'/media/system/js/caption.js']);
unset($doc->_scripts[JURI::root(true) . '/media/system/js/mootools.js']);
unset($doc->_scripts[JURI::root(true).'/media/system/js/mootools-core.js']);
unset($doc->_scripts[JURI::root(true).'/media/system/js/mootools-more.js']);
=> Edit to:
// Remove default mootools
/* unset($doc->_scripts[JURI::root(true) . '/media/system/js/core.js']);
unset($doc->_scripts[JURI::root(true) . '/media/system/js/modal.js']);
unset($doc->_scripts[JURI::root(true).'/media/system/js/caption.js']);
unset($doc->_scripts[JURI::root(true) . '/media/system/js/mootools.js']);
unset($doc->_scripts[JURI::root(true).'/media/system/js/mootools-core.js']);
unset($doc->_scripts[JURI::root(true).'/media/system/js/mootools-more.js']);
*/
Thanks