Ok, I feel quite stupid now, cause this must be very simple:

This returns true:
Object::has_extension('ErrorPage',"ErrorPageDecorator");

But the canCreate() and canEdit() in the Decorator does not change anything in the CMS...

<?php
// _config.php : Object::add_extension('ErrorPage','ErrorPageDecorator');
class ErrorPageDecorator extends DataObjectDecorator{
	
	function __construct() {
		parent::__construct();
	}
	
	function canCreate(&$member) {
		//return Permission::check("ADMIN");
		return false;
	}
	
	function canEdit(&$member){
		return false;	
	}
}

What am i missing?

OK,

Seems this does work when not logged in as admin....