I hope this will help other people out there:
If you get 404 errors on php pages and the error.log of Hiawatha shows following errors:
/var/www/test/public/phptest.php|Unable to open primary script: /var/www/test/public/phptest.php (No such file or directory)
Make sure you have set the chroot setting correct in php-fpm.conf!
chroot = /