Could you try again the index.php page after rebooting your NAS. I've already had strange reports about the need to reboot the webserver to make it work.
First I should have updated config_default.php, FBReaderJ is now fully compliant to Opensearch. So no need to enable dirty hacks for this software.
With FBReaderJ, could you try to reload the catalog (top right).
Could you also test with cops-demo.slucas.fr/feed.php