2.4.13 server issues post upgrade to CentOS clients?

Hi all.
I’ve struck with one specific server to client(s) running CentOS 7, that the server no longer gets a response from the client for backing up. Either it states on the server “Backup Failed” or shows that it is offline.
I tried to then move the CentOS 7 client to a new QNAP NAS running urBackup 2.4.13 and reinstalled the client package with the same result.
Another CentOS box in the same situation as above was able to be seen on the NAS as online but wouldn’t back up.
The initial server seems fine backing up Debian, Ubuntu and Windows clients fine.
The primary server is on one site with VPN’s to the client sites and the NAS Server is on another site. Routing, etc is fine and there is no port-specific firewalls causing issues.

Anyone else struck this?