All .htaccess-Files are correct. The PHP-specs are met too.
However, I’m getting the error, that the page/page-ID doesn’t exist. The pages are editable in the backend though.
Also, the pages are visible and concerning the folder structure, the folder-names start with a digit.
Actually, I found the culprit. I manually created a folder inside the content folder called “5-Impressum” however in the PHP-snippet, I referred to it as “impressum”. So it came down to a uppercase/lowercase-mistake.