If your website is hosted on All-Inkl and any of the following does not work (i.e. only shows the web server's 404 response):
then an orphaned .htaccess file in a parent directory of your Contao instance may be the culprit, likely in the root of your (configurable) webspace. So check all the parent directories of the Contao instance for such a rogue .htaccess file and delete it.
This affects Contao 4 and higher.