For the site I want to mirror, the login data is stored in 
a cookie, so that I do not need to log in again when 
beginning a new session. 

I copied COOKIES.TXT from the Netscape folder to the 
WinHTTrack project folder, started HTTrack, excluded the 
logout link in Scan Rules and re-mirrored the site.

None of the protected web pages were downloaded and as soon 
as HTTrack finished, COOKIES.TXT got overwritten. All 
cookies collected from the many web sites I visited in 
recent times are gone. Just the cookie from the downloaded 
site is present with modified content (see samples at 
bottom of message). It seems HTTrack ignored the original 
file and replaced it with its own generated cookies file.

The result is the same when I use "* Update existing 
download". 

Why does it not work?
----------------------------------------------
Original file copied to project folder:

# Netscape HTTP Cookie File
# <http://www.netscape.com/newsref/std/cookie_spec.html>
# This is a generated file!  Do not edit.
kcookie.netscape.com	FALSE	/	FALSE	4294967295
....
www.byte.com	FALSE	/	FALSE	1358281290
	jive.authorization.autologin
	LkIjFkIkMjKkLhDnDhPhEiNhHhFnJmLmMhFhChHiJhImCiHhLlAh
GhIiMhClEkIhCjPhIlDiKhJhPvEtDvFsEwMtOvNuEtMwCsNwIsHwPwGtEwDu
DvHvHtEtEuBvJrEuNrCvNrBwGv

New file in project folder:

# HTTrack Website Copier Cookie File
# This file format is compatible with Netscape cookies
www.byte.com	TRUE	/	FALSE	1999999999
	JSESSIONID	MU3JZBNZHEXWSCQAAAGSFEQ