SELECT COUNT(DISTINCT p.poster_id) AS count, f.forum_id AS f_id, f.forum_name AS f_name FROM phpbb_posts p, phpbb_forums f WHERE p.forum_id = f.forum_id AND p.post_approved = 1 AND f.forum_type = 1 GROUP BY f.forum_id, f.forum_name ORDER BY count DESC LIMIT 10
As far as I can see, the result of query is just too big to be processed by your SQL server. The query needs to be split up in several queries like on the miscellaneous statistics page. This is currently not possible but it will be possible in one of the next versions. Right now I can only suggest you to turn off that part of the statistics.