QueryMonitor Fatal error: Cannot declare class QM_PHP, because the name is already in use

Case: Staging and production copies of a WordPress 5.4 website on the same server. Activating QueryMonitor 3.9.0 on Staging works, but throws `Fatal error: Cannot declare class QM_PHP` on Production.

Solution

Remove the file: /wp-content/db.php

Source

 

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