bbPress.org

Interestingly, I copied and pasted your SQL query from above, and used it on my installation unchanged, and it did not error out, it just return the expected results.

SELECT t.*, MIN(p.post_id) as post_id, p.post_text, 0 AS search_score FROM bb_topics AS t JOIN bb_posts as p ON ( t.topic_id = p.topic_id ) WHERE t.topic_status = '0' AND p.post_status = '0' AND ( (t.topic_title LIKE '%ja%') OR p.post_text LIKE '%ja%' ) GROUP BY t.topic_id ORDER BY t.topic_time DESC LIMIT 20

See the results here.

This is an old bbPress installation as well, 1.0-alpha from over a year ago.