If Moodle is running on multiple web server tiers and opcache is enabled when the cache configuration is changed the settings only take effect on the webserver the change was made on.

This occurs because the configuration is cached in a php file <moodledata>/muc/config.php

Opcache must be cleared on all the servers for the changes to take effect correctly.

How to reproduce

Pre-requisites

• You must have a Moodle installation with two webservers that are load balanced
• they must share the Moodledata directory
• they must share the shared database
• opcache must be enabled on both servers

Steps

• Login to Moodle and go to Site administration > Plugins > Caching > Configuration
• Add a new cache store instance
• Make sure you change the server you are load balance to. If the load balancer does not keep you on the same server by repeatedly refreshing the browser you should see the new cache store instance appear and disappear from the configuration page.