Forbidden – Private List Deny

We are running this plugin on a network, WP Multisite, and the configuration is used for the entire network of sites.

We have a client that has a contact form for hiring new employees. They’ve had multiple people receive this message:

*** Forbidden. Private list deny. ***

This is preventing people from being able to apply. Do you have any idea what could be causing this?

Here’s an image of the error – https://ibb.co/VYGyY3X