Symptoms

1. When issuing a command from the WHMCS portal (create client, hosting package) it says:

   Module Command Error
   Unable to check subscription in PPA. Malformed response from PPA. Please make sure the PPA management node is available and the IP address of your WHMCS server is added to the list of allowed networks for PPA API. You can check this at PPA > System > Settings > Public API
   

2. Following error appears in /var/log/poa.debug.log on PA Management node:

   ERR [openapi-firewall:f5782b70 1:2549:f5782b70 lib]: [iqxmlrpc-server] iqxmlrpc::Https_server_connection: SSL: tlsv1 alert unknown ca
   

3. WHMCS website is working on custom PHP and openssl with customized ssl ciphers support.

Cause

The behavior was considered as internal software issue with id #PPA-2873.

Resolution

The fix is included in Plesk Automation MU#23.

Instructions on installing PA updates available at How to install PA updates.