Page not found error (error404)
ImpressPages throws an ipRouteAction job. If nor the core nor any of the plugins catches this job and returns a response, a page not found page is being shown.
Add 404.php file to your theme to customize the layout of 404 page.
Custom error404 page
You can place your own content on 404 page. Create a regular page in Admin -> Pages and set its alias to error404 . Now this page will be used instead of the default one.