after you capture the login, you reset the browser so it's back to normal right
?
then it dies when HTTrack takes over.

what about cookies ? are you setting HTTrack to use the cookies for that
browser too ? use the browser and see if the site uses any cookies.

also, what kind of login is it? just HTML form ? if so then you should be able
to get HTTrack to login and do the download. I've done it on other sites that
use an HTML form/INPUT to handle the login. I just made sure the User Agent is
the same as my browser, and the cookies file is set correctly.