I'm no expert I'm afraid, but what happens if you add the
www/kaufmich.com/forum/ URL separately? E.g. First the login page and then the
forum URL.

This will rely on the login being the same for the site and the login, and for
HTTrack to keep spidering with the same login information.

I'm not sure about excluding elements as it's not something I've had to do,
but if you look through the documentation and keep experimenting I'm sure you
can get it to work.

Good luck.