Okay, getting scared now. I've just run the plugin from the CMD line. But I'm getting closer (I think).
Code:
File "calibre_plugins.fanfictiondownloader_plugin.fanficdownloader.adapters.adapter_fictioningnet", line 109, in extractChapterUrlsAndMetadata
TypeError: object of type 'NoneType' has no len()
Line 109 is, I think, about the Story Title?
Code:
self.story.setMetadata('title',stripHTML(a)[:(len(a.string)-3)])
In case anyone doesn't know how to do this, I used this code (you'd replace the URL with your target site, of course):