The cleaner page showed “media cleaner database not ready” (paraphrase) before clicking reset. After clicking reset that error went away but these appeared in the error log:
[27-Apr-2018 12:29:30 UTC] WordPress database error Table 'asd_wp.asdwp_mclean_scan' doesn't exist for query SELECT COUNT(*) FROM asdwp_mclean_scan WHERE ignored = 0 AND deleted = 0 made by require_once('d:\path\to\wp\wp-admin\admin.php'), do_action('media_page_media-cleaner'), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, Meow_WPMC_Core->wpmc_screen
[27-Apr-2018 12:29:30 UTC] WordPress database error Table 'asd_wp.asdwp_mclean_scan' doesn't exist for query SELECT SUM(size) FROM asdwp_mclean_scan WHERE ignored = 0 AND deleted = 0 made by require_once('d:\path\to\wp\wp-admin\admin.php'), do_action('media_page_media-cleaner'), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, Meow_WPMC_Core->wpmc_screen
[27-Apr-2018 12:29:30 UTC] WordPress database error Table 'asd_wp.asdwp_mclean_scan' doesn't exist for query SELECT SUM(size) FROM asdwp_mclean_scan WHERE ignored = 0 AND deleted = 1 made by require_once('d:\path\to\wp\wp-admin\admin.php'), do_action('media_page_media-cleaner'), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, Meow_WPMC_Core->wpmc_screen
[27-Apr-2018 12:29:30 UTC] WordPress database error Table 'asd_wp.asdwp_mclean_scan' doesn't exist for query SELECT COUNT(*) FROM asdwp_mclean_scan WHERE ignored = 1 made by require_once('d:\path\to\wp\wp-admin\admin.php'), do_action('media_page_media-cleaner'), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, Meow_WPMC_Core->wpmc_screen
[27-Apr-2018 12:29:30 UTC] WordPress database error Table 'asd_wp.asdwp_mclean_scan' doesn't exist for query SELECT COUNT(*) FROM asdwp_mclean_scan WHERE deleted = 1 made by require_once('d:\path\to\wp\wp-admin\admin.php'), do_action('media_page_media-cleaner'), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, Meow_WPMC_Core->wpmc_screen
[27-Apr-2018 12:29:30 UTC] WordPress database error Table 'asd_wp.asdwp_mclean_scan' doesn't exist for query SELECT id, type, postId, path, size, ignored, deleted, issue
FROM asdwp_mclean_scan
WHERE ignored = 0 AND deleted = 0 AND path LIKE '%%'
ORDER BY time
DESC LIMIT 0, 25 made by require_once('d:\path\to\wp\wp-admin\admin.php'), do_action('media_page_media-cleaner'), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, Meow_WPMC_Core->wpmc_screen
However, it seems to be fixed now because subsequent click of reset does not re-produce the errors.