Nope, Fimfic has 'helpfully' changed again.
Fimfic's API is what's reporting 'Invalid story id'. See:
http://www.fimfiction.net/api/story.php?story=5588
I can work around this one by checking the error status from the API after looking for different error states in the scraped text instead of before. Try the attached version.
UPDATE Jan 28 2013 - Remove obsolete beta versions