I found the fix on here:

<https://forum.httrack.com/readmsg/29365/index.html>

It took me a little while to understand, but basically the only thing I had to
do was replace and add some missing cookies. I compared httrack's cookies from
catchURL to the cookies I exported using Export Cookies from
<https://addons.mozilla.org/en-US/firefox/addon/export-cookies/> and found
that httrack did not save all of the cookies that firefox used to stay logged
in. Also, some of the cookie values were different, though I'm not sure if the
different cookie values were significant. I added the new cookies and replaced
the ones with different values, and it suddenly started getting past the
initial welcome page!