WordPress database error Illegal mix of collations

Hi there!

I love this plugin, but when I upload an image to wordpress, eventhough everything seems to work fine and the image is optimized, I do get this error in our error log :

WordPress database error Illegal mix of collations (utf8mb4_unicode_ci,IMPLICIT) and (utf8mb4_unicode_520_ci,IMPLICIT) for operation ‘=’ for query SELECT COUNT(DISTINCT p.ID)
FROM wp_posts p
WHERE NOT EXISTS (SELECT *
FROM wp_rio_process_queue
WHERE p.ID = object_id AND p.post_type = item_type)
AND p.post_type = ‘attachment’ AND p.post_mime_type IN ( ‘image/jpeg’, ‘image/gif’, ‘image/png’ ) made by wp_ajax_upload_attachment, media_handle_upload, wp_generate_attachment_metadata, apply_filters(‘wp_generate_attachment_metadata’), WP_Hook->apply_filters, WRIO_Media_Library::optimize_after_upload, WRIO_Media_Library->optimizeAttachment, WRIO_Image_Statistic::get_instance, WRIO_Image_Statistic->__construct, WRIO_Image_Statistic->load, WRIO_Image_Statistic::get_unoptimized_count, W3TC\DbCache_Wpdb->query, W3TC\DbCache_WpdbInjection_QueryCaching->query, W3TC\_CallUnderlying->query, W3TC\DbCache_Wpdb->query, W3TC\DbCache_WpdbInjection->query, W3TC\DbCache_Wpdb->default_query

Is there anything that can be done ?

Thanks!
Jonathan