Hello @dcbaker ,
I was able to recreate the reported behavior. It was caused by the fact that the Content Control plugin is trying to redirect the protected page to the default WordPress login page, while there is a custom admin page configured by our plugin.
To resolve the issue, you should configure the Content Control plugin to redirect the visitors who access protected pages to the same custom login page you created with the security plugin.
You can do it in wp-admin -> Settings-> Content Control -> click Edit under the rule you created -> Protection:
Best Regards,
Kuzman Stoyanov