If you’re getting a white screen then there must be an error. It could be a parse error in a file, which I’m not sure if WP DEBUG can report on or not.
My next suggestion would be to turn on standard php error reporting rather than using WP DEBUG. Either by changing php.ini, or in your .htaccess file if you can do that.
I would usually add the following to my .htaccess file, but this may not work for you.
php_value display_errors On
php_value error_reporting 6143
If you can’t do either of the above then you need to open the first file loaded and add the following line to the top of the file. For updating or editing a post is would be /wp-admin/post.php
ini_set('display_errors', 1);
