Database errors created by Media Cleanup?

A number of images are showing up as blank in the media library, but when I go into edit them they are actually there and, usually, can be re-inserted. I’ve also run regenerate thumbnails and a few things, but Media Cleaner seems to be the most likely culprit.

I’m got the following warnings:

[13-May-2020 05:06:02 UTC] PHP Warning: Use of undefined constant DB_USER – assumed ‘DB_USER’ (this will throw an Error in a future version of PHP) in /home3/travgal/public_html/wp-content/plugins/query-monitor/wp-content/db.php on line 139
[13-May-2020 05:06:02 UTC] PHP Warning: Use of undefined constant DB_PASSWORD – assumed ‘DB_PASSWORD’ (this will throw an Error in a future version of PHP) in /home3/travgal/public_html/wp-content/plugins/query-monitor/wp-content/db.php on line 139
[13-May-2020 05:06:02 UTC] PHP Warning: Use of undefined constant DB_NAME – assumed ‘DB_NAME’ (this will throw an Error in a future version of PHP) in /home3/travgal/public_html/wp-content/plugins/query-monitor/wp-content/db.php on line 139
[13-May-2020 05:06:02 UTC] PHP Warning: Use of undefined constant DB_HOST – assumed ‘DB_HOST’ (this will throw an Error in a future version of PHP) in /home3/travgal/public_html/wp-content/plugins/query-monitor/wp-content/db.php on line 139

The code on line 139 is:
// phpcs:ignore WordPress.WP.GlobalVariablesOverride.Prohibited
$wpdb = new QM_DB( DB_USER, DB_PASSWORD, DB_NAME, DB_HOST );

I posted this earlier with more detail of what was happening when I thought it was a media library problem. I’ve eliminated that and a few other possible culprits since, but this has more information on what I am getting when I use the Media Library either from the admin panel or editing a post: https://wordpress.org/support/topic/some-images-missing-in-media-library/