hmmm, after changing $subsite = $this->owner->Subsite(); to $subsite = Subsite::currentSubsite(); it is working now.

	function updateCMSFields(&$fields) {
		if($this->owner->MasterPageID) $fields->insertFirst(new HeaderField('This page\'s content is copied from a master page: ' . $this->owner->MasterPage()->Title, 2));
		
		// replace readonly link prefix
		//$subsite = $this->owner->Subsite();
		$subsite = Subsite::currentSubsite();
		if($subsite && $subsite->ID) {
			$baseUrl = 'http://' . $subsite->domain() . '/';
			$fields->removeByName('BaseUrlLabel');
			$fields->addFieldToTab(
				'Root.Content.Metadata',
				new LabelField('BaseUrlLabel',$baseUrl),
				'URLSegment'
			);
		}