This error shows up in the PHP log file.
WordPress database error Illegal mix of collations (utf8mb4_unicode_520_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation 'like' for query SELECT `c`.`comment_author_email`, `c`.`comment_author`, COUNT(`c`.`comment_ID`) AS `count`, IFNULL(`s`.`count`, 0) AS `scount`, IFNULL(`fi`.`count`, 0) AS `ficount`, IFNULL(`fw`.`count`, 0) AS `fwcount`, MAX(`c`.`comment_date_gmt`) AS `last_date` FROM `wp_comments` AS `c` LEFT JOIN (SELECT `email`, COUNT(`email`) AS `count` FROM `wp_wc_comments_subscription` WHERE `confirm` = 1 GROUP BY `email`) AS `s` ON `s`.`email` LIKE `c`.`comment_author_email` LEFT JOIN (SELECT `follower_email`, COUNT(`follower_email`) AS `count` FROM `wp_wc_follow_users` WHERE `confirm` = 1 GROUP BY `follower_email`) AS `fi` ON `fi`.`follower_email` LIKE `c`.`comment_author_email` LEFT JOIN (SELECT `user_email`, COUNT(`user_email`) AS `count` FROM `wp_wc_follow_users` WHERE `confirm` = 1 GROUP BY `user_email`) AS `fw` ON `fw`.`user_email` LIKE `c`.`comment_author_email` WHERE `c`.`comment_approved` = '1' AND `c`.`comment_type` NOT IN('order_note') GROUP BY `c`.`comment_author_email`, `c`.`comment_author` ORDER BY `count` DESC LIMIT 7 OFFSET 0; made by do_action('wp_ajax_wpd_stat_user'), WP_Hook->do_action, WP_Hook->apply_filters, WpdiscuzHelperAjax->wpd_stat_user, WpdiscuzDBManager->getActiveUsers
