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Hello @francescocortese,
You can resolve this by editing the following file, replace line 880 and 881 with the code given below
wp-content/plugins/tutor/includes/tutor-general-functions.php
We will surely fix this in our next update.
$tags = array( 'input', 'style', 'script', 'select', 'form', 'option', 'optgroup', 'iframe', 'bdi', 'source', 'a' );
$atts = array( 'min', 'max', 'maxlength', 'type', 'method', 'enctype', 'action', 'selected', 'class', 'id', 'disabled', 'checked', 'readonly', 'name', 'aria-*', 'style', 'role', 'placeholder', 'value', 'data-*', 'src', 'width', 'height', 'frameborder', 'allow', 'fullscreen', 'title', 'multiple', 'tutor-hide-course-single-sidebar' );