I think I had the same problem at some point. Jason suggested to run the following query, and the problem was fixed:
Maybe this also helps for you.
Ivan
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Iyengar Yoga Resources / GT Plugins
Code:
ALTER TABLE gforum_Post ADD INDEX p_rfl (post_root_id, forum_id_fk, post_latest_reply)Ivan
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Iyengar Yoga Resources / GT Plugins