Hi Giacomo,
A quick fix for the empty 404 page problem is to add lower priority to the URE Pro’s post filter. You can open user-role-editor-pro/pro/includes/classes/content-view-restrictions-posts-list.php and replace line #17
add_action('pre_get_posts', array($this, 'hide_prohibited_posts'));
with this version
add_action('pre_get_posts', array($this, 'hide_prohibited_posts'), 100);
A reason of this conflict the woffice theme applies its posts restrictions filter even if restricted posts list is empty. Applying this filter theme overwrites the same filter set by URE Pro little earlier. So a restricted page exists for WordPress and WP tries to show it, but with empty content as URE does not allow to view content of restricted post here too.