Empty page instead of 404 error when url contains channel slug
As Brad pointed out in the comments on my question I had to take a look in the custom route's defined in the Craft CP. My client tried to make a custom route which looked like this:



This caused for every url containing one "/" to use that template that unfortunately actually was a template that's why it wouldn't give a 404.

Original Comment:

Maybe an .htaccess rewrite rule interfering? Do you have any custom routes defined (in the Craft CP or in config/routes.php)? – Brad Bell