Latest update to Meta Box AIO took down one of our key websites ("critical error").
Investigation showed that the latest version of Meta Box AIO was the culprit.
Here's an excerpt of the error log (some details omitted for confidentiality):
PHP Fatal error: Uncaught Error: Class 'MetaBox\Columns\Processor' not found in /[REMOVED]/[REMOVED]/public_html/wp-content/plugins/meta-box-aio/vendor/meta-box/meta-box-columns/meta-box-columns.php:30
Rolling back to an earlier version of the plugin resolved the issue for us.