I searched for "PayPal" but didn't find anything, so I'm making a new post. Would the SCP firewall block PayPal IPN notifications? One of my client sites suddenly stopped receiving the notices. I don't recall seeing anything in the log. Just trying to eliminate that as a possible cause. I'll run another test and check the log again.
The firewall could block Paypal if its IP addresses are (geo)blocked or there is a false positive. In the second case you should see a log entry in Securitycheck Pro everytime this happens, and it's usually solved enabling the 'Easy config' feature (I always recommend to enable this option on ecommerce websites). In the first case you will not see anything in logs unless you enable it into WAF configuration -> Logs tab (Enable "Add Geoblock logs" and "Add access attempts").