I have the same problem, wondering if there has been any 
progress on this since original post.

This is not a phpBB issue - setting it to automatically log 
me in each time works fine when viewing various pages 
in "hidden" forums in my IE browser.

When HTTrack attempts to retrieve these pages, it diverts 
to the login page as if the cookie does not exist (or as if 
HTTrack isn't using the cookie properly).