> (beautyleg.com) for a while, and downloaded new sets
> with HTTrack flawlessly.

> with "Authentication Required" in the error log. Is

Once it authenticates, it shouldn't require another.
First make sure you're mirroring www.beautyleg.com as that's where some of the
links go.
Also I'd start at the members area so the cookie is initially set.
I'd also set the browser ID to something other than httrack.