Firewall preventing local loopback

I'm trying to set up Wordpress Multisite with the Multisite Provisioning Plugin from WPMUDev and WHMCS. My setup is:

nginx

PHP-FPM

WHMCS directory: domain.com/clients

The plugin Multisite Provisioning Plugin author has said:

"A common problem we've found if the WHMCS and Wordpress sites are run on the same host is that some hosts in the name of security firewall local loopback. That means that one site on the host server can't talk to another site on the same host server."

I am very new to linux, so I'm not sure what to do with this information. Can someone tell me how to check if I have a security firewall that is preventing local loopback? And even better, how to fix it if I do?

Thanks!