I'm trying to get all fanfictions from an author the same way and I run into
the same problem, but I'm not quite sure what the solution was. (since you
said "it worked!" I assume you found one)

Was it putting those filters?-*
+m.fanfiction.net/anime/Sailor_Moon/*
+m.fanfiction.net/s/*

Or something else? (I'm very new on using Httracks and I barely touched the
filters so far, I use the limits in the interface instead)