It's working fine for me. I tried it on Ubuntu 12.04.1 LTS with calibre 0.8.68 and FFDL 1.6.8. Although it is a story that requires you to answer that yes, you're an adult.
Giving multiple chapter URLs to the same story won't do anything useful. Either they'll be ignored or they'll all normalize to the same story URL.
You can get more debug output by running calibre from command line as 'calibre-debug -g'.