> So I copied the Mozilla Firefox « cookies.txt » to the 
root
> of my HTTrack project...

Can you try to copy them in the project folder itself ? I 
think the FAQ is bogus (the 'cookie_load
(opt.cookie,opt.path_log,"cookies.txt")' only concerns the 
project log path, that is, not the root folder).
Do do that: select 'save settings' instead of firing the 
download, copy the cookies, and restart httrack to begin 
the mirror.