We have made a fix to admin_menu_submenu.html template after release 3.3 has been issued.
 
Description.
An empty page has been displayed while attempting to edit a secondary menu.
 
Solution.
Please, download an updated version of the template from here:
http://www.viart.com/downloads/admin_menu_submenu-3.3.zip
Then extract the template admin_menu_submenu.html into the templates/admin/ folder replacing an existing one.