[Vili]

<?php get_header(); ?>

<?php global $forumpage;

$forumpage = TRUE; ?>

…should be the other way around:

<?php global $forumpage;

$forumpage = TRUE; ?>

<?php get_header(); ?>

You need to set the $forumpage boolean before you call the header, or you won’t be able to use it there.

Alternatively, forget about $forumpage, and use a global boolean such as $bbdb that bbPress uses anyway, as suggested by fel64.