Hello Hans,
I see what you mean. If you are talking about a page like http://staging.toerfietsleusden.nl/sdafsd where I enter the wrong URL and it shows “Not Found – The requested URL was not found on this server.” This is not from the theme, it’s from your server. Please check in your server there might be a default 404.html or 404.php. Once you remove that then it should show our theme 404 page like this https://catchthemes.com/demo/travelore-pro/sdafsd.
Once you do that, you can use the widget to change the content of the 404 page. There is widget areas called “404 Page Not Found Content ”
Regards,
Sakin
