I have had this error on two separate hosting providers. It was originally happening on the host I developed this site on, and today I have migrated to a new host (Kinsta) and it is still happening there.

This error happens when the BuddyPress Moderation plugin is enabled:

2019/11/04 23:54:10 [error] 13703#13703: *23268 FastCGI sent in stderr: "PHP message: WordPress database error You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ',,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,' at line 1 for query SELECT q.item_id FROM wp_bpmts_moderation_queue as q WHERE q.item_type='bp_group' AND q.item_id IN (,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,) AND (q.item_id IN (SELECT item_id FROM wp_bpmts_moderation_queue WHERE item_type= 'bp_group' AND is_hidden = 1)) made by do_action('wp_ajax_media_filter'), WP_Hook->do_action, WP_Hook->apply_filters, bp_nouveau_ajax_object_template_loader, apply_filters('bp_nouveau_object_template_result'), WP_Hook->apply_filters, bp_nouveau_object" while reading response header from upstream, client: 0.0.0.0, server: HIDDEN.com, request: "POST /wp-admin/admin-ajax.php HTTP/1.0", upstream: "fastcgi://unix:/var/run/php7.3-fpm-hidden.sock:", host: "HIDDEN.com", referrer: "https://HIDDEN.com/members/joesmith/photos/"

It happens many times a day.