Hi,
The problem is in TinyMCE that comes with WordPress, if you go to the classic editor the TyniMCE doesn’t load.
WordPress Is trying to load this file http://www.photonix.it/wp/wp-includes/js/tinymce/langs/it.js that doesn’t exist.
Or something wrong with your WordPress install or with the language settings. Please try to updated your WordPress install or change back the language to the default just to check if it works and it’s a language/translation problem.
Let us know the result.
Best Regards,
Rui