> i ve downloaded the version available from the site

The site uses form-based authentication: you need to erase 
your existing cookies, go to the login page, fill the form 
without submitting it, launch HTTrack and use the "Add 
URL/Capture URL" mechanism, and then launch the mirror. 
This may work, or not, depending on the authentication 
complexity (javascript ? multiple redirections with 
cookies ?)