Ok, so the remaining question is how got this plugin activated. Did you or your client activate it or did the ISPConfig updater activate it. The ISPConfig updater and installer contain code that prevents that both plugins get activated at the same time even when apache and nginx are both installed. I tested this here with Debian by installing a nginx server beside the existing apache server and the updater enabled only the apache plugin and skipped the nginx plugin. So only one of the plugins was enabled at a time, never both together.
I will redo the test with centos to see if that makes any difference.