magento2.2 – The new route does not work (2.2)

I'm frustrated. I can not add an ajax route in Magento 2.2, no matter what I try, I get a 404.

routes.xml:



    
        
            
        
        
            
        
    

The controller is next to the controllers working at [module path]/Controller/Adminhtml/Ajax/Index.php and looks like this:

// the namespace is defined here.
class Index expands  Magento  Backend  App  Action
{
protected $ resultPageFactory;

public service __construct (
 Magento  Backend  App  Action  Context $ context,
 Magento  Framework  View  Result  PageFactory $ resultPageFactory
) {
die ("foo");
}

public function execute ()
{
die ("foo");
}
}

/ admin / schematics_ajax / index shows a 404

I'm pretty sure I've followed https://devdocs.magento.com/guides/v2.2/extension-dev-guide/routing.html pretty much exactly. What do I miss?