I just updated to the latest svn version 3981 too, in an attempt to resolve a fog pxe boot issue. And I’m also getting the same blank screen when attempting to update the schema.

Looking in the http error log I see the following error:
PHP Fatal error: Call to a member function get() on boolean in /var/www/html/fog/lib/plugins/accesscontrol/hooks/RemoveUpload.hook.php on line 28