TomyJan you need to access the website via the exact same URL defined in config.php (that will be the one used during installation by default)
From the error message, it looks like you set the www. variant of the URL as the website URL, so you need to use that all the time and setup redirects so users don't use the wrong URL.
If you wanted to use the no-www variant instead, just edit config.php and insert the new URL.
If you don't want or cannot setup redirects using your webserver, you can use the Canonical URL extension https://discuss.flarum.org/d/19307-canonical-url-redirect