Problem with “Darren’s Related Posts plugin”

I’ve installed Darren’s Related Posts plugin and it seemed to work well, but today (only on one post) i get an error telling:

WordPress database error: [The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET SQL_MAX_JOIN_SIZE=# if the SELECT is okay]
SELECT x.post_title AS title, p.post_id FROM wp_posts x, wp_postmeta p, wp_postmeta p0, wp_postmeta p1, wp_postmeta p2, wp_postmeta p3, wp_postmeta p4, wp_postmeta p5, wp_postmeta p6 WHERE x.ID = p.post_ID AND x.post_date <= NOW() AND x.post_status = 'publish' AND p.post_id != '65' AND p.meta_key = 'keywords' AND p.post_id = p0.post_id AND p.meta_key = p0.meta_key AND p0.meta_value = 'successo' AND p.post_id = p1.post_id AND p.meta_key = p1.meta_key AND p1.meta_value = 'raggiungimento obiettivi' AND p.post_id = p2.post_id AND p.meta_key = p2.meta_key AND p2.meta_value = 'crescita personale' AND p.post_id = p3.post_id AND p.meta_key = p3.meta_key AND p3.meta_value = 'alimentazione' AND p.post_id = p4.post_id AND p.meta_key = p4.meta_key AND p4.meta_value = 'disciplina' AND p.post_id = p5.post_id AND p.meta_key = p5.meta_key AND p5.meta_value = 'bambini' AND p.post_id = p6.post_id AND p.meta_key = p6.meta_key AND p6.meta_value = 'efficacia' GROUP BY p.post_ID ORDER BY x.post_date DESC LIMIT 4

This error message is displayed immediately AFTER the first related post. The post is https://www.strategievincenti.net/2007/08/apprendimento-e-raggiungimento-degli-obiettivi-3-tecniche-da-imparare-dai-bambini/.

I’m totaly PHP and MySQL ignorant, so please help me.

Thanks in advance

ciao
alexander