For me personally it would make sense to try to install the latest version of the core forum script (without any custom modifications) and start from there.. Then you could be adding modifications and after each modification you could test it and have a chance to find what exactly causes the problem.

For me personally it would make sense to try to install the latest version of the core forum script (without any custom modifications) and start from there

Yes, I would do recommend the same.

You did some custom changes and afterall it's difficult to say what exactly could be the reason. As you see default POSTS table works fine.

I couldn't restart it yet but today I tried something different... just to share:

without the "where forum_id"
SELECT topic_id
FROM forumlv_topics
ORDER BY topic_last_post_id DESC
LIMIT 500 , 50
50 rows in set (0.02 sec)

SELECT topic_id
FROM forumlv_topics
ORDER BY sticky DESC,topic_last_post_id DESC
LIMIT 550 , 50
50 rows in set (2.95 sec)


Maybe too many repeated values (sticky=0 and forum_id=1) is the cause... I'll keep you guys informed... thanks =)