@lmagnif, Yeah I have the same issue, Looks like it’s to do with “ABSPATH” If you go to the top of widgets.php and make a similar mod:
/** WordPress Administration Widgets API */
//require_once(ABSPATH . ‘wp-admin/includes/widgets.php’);
require_once(‘includes/widgets.php’);
It will display the page.
Not an ideal solution but it works.