function fix_pagination($args) {
	global $wp_query,$bbp;
	$max_pages = $bbp->topic_query->max_num_pages;
	$page = $bbp->topic_query->paged;
	$mybase = get_permalink($wp_query->post->ID);
	$args = array (
			'base'      => $mybase.$wp_rewrite->pagination_base.'/%#%/',
			'format'    => '',
			'total'     => $max_pages,
			'current'   => $page,
			'prev_text' => '←',
			'next_text' => '→',
			'mid_size'  => 1
		);
	return $args;
}
add_filter('bbp_topic_pagination','fix_pagination');

For some reason bbPress seems to use the base domain name (rather than the current forum) as the URL to which it appends /page/2 (for example) in its pagination. This seems to fix it.

Fix bbPress pagination

PHP