My hunch is that Caddy isn’t finding an index.php file to rewrite to. Do you have an index.php file at /var/www/demo.orbitale.io/repos/faker_online/public/index.php?
Can you post your full debug logs for a request you expect to work? Ideally it should show information about the rewrites happening.
I wrote a pretty in-depth explanation about a week ago on how the php_fastcgi directive works. Read this and see if the points align with the behaviour you’re seeing:
If you remove file_server, then there’s no directive that ends up handling the request, so you encounter Caddy’s default behaviour, which is to respond with an empty HTTP 200. That’s because “technically it worked as configured”, you didn’t configure Caddy to actually do anything with the request so it’s not actually an error on Caddy’s part.
I think in this case, you’ll definitely need this, otherwise Caddy won’t resolve the symlink before passing the request onto your php-fpm service.