FIXED
Received a successful IPN from PayPal today. Here are 3 things to check when using Akeeba Admin tools for a firewall solution.
1. Do not use the Administrator Password Protection
2. disable the following Web Application Firewall options:
- "Cross Site Scripting block (XSSShield)" as it may cause issues with a minority of IPN requests
- "CSRF/Anti-spam form protection (CSRFShield)" as it will get triggered by POST requests originating from a remote server (they have a blank referrer or a referrer not belonging to your site)