dozy

ok, found my mistake.
added

add_action( 'pre_get_posts', 'lmg_orderby' );
	function lmg_orderby( $query ) {
		if( ! is_admin() )
			return;

		$orderby = $query->get( 'orderby');

		if( 'startzeit-datum' == $orderby ) {
			$query->set('meta_key','startzeit-datum');
			$query->set('orderby','meta_value_num');
		}
		if( 'endezeit-datum' == $orderby ) {
			$query->set('meta_key','endezeit-datum');
			$query->set('orderby','meta_value_num');
		}
	}