FIND
$subforum_result = $db->query('SELECT forum_desc, forum_name, id, last_post, last_post_id, last_poster, moderators, num_posts, num_topics, redirect_url FROM '.$db->prefix.'forums WHERE parent_forum_id='.$id.' ORDER BY disp_position') or error('Unable to fetch sub forum info',__FILE__,__LINE__,$db->error());
REPLACE WITH
$subforum_result = $db->query('SELECT f.forum_desc, f.forum_name, f.id, f.last_post, f.last_post_id, f.last_poster, f.moderators, f.num_posts, f.num_topics, f.redirect_url, p.poster_id AS last_poster_id FROM '.$db->prefix.'forums LEFT JOIN '.$db->prefix.'posts ON (f.last_post_id=p.id) WHERE parent_forum_id='.$id.' ORDER BY disp_position') or error('Unable to fetch sub forum info',__FILE__,__LINE__,$db->error());
FIND
$last_post = '<a href="viewtopic.php?pid='.$cur_subforum['last_post_id'].'#p'.$cur_subforum['last_post_id'].'">'.format_time($cur_subforum['last_post']).'</a><br /> <span class="byuser">'.$lang_common['by'].' '.pun_htmlspecialchars($cur_subforum['last_poster']).'</span>';
REPLACE WITH
$last_post = '<a href="viewtopic.php?pid='.$cur_subforum['last_post_id'].'#p'.$cur_subforum['last_post_id'].'">'.format_time($cur_subforum['last_post']).'</a><br /> <span class="byuser">'.$lang_common['by'].' <a href="profile.php?id='.$cur_subforum['last_poster_id'].'">'.pun_htmlspecialchars($cur_subforum['last_poster']).'</a></span>';