PSA: If Master can't reach Backup, double check "Listen Interfaces" on Backup

[Grossartig]

Took me a while to figure out why my Master OPNsense box couldn't reach the Backup box via XMLRPC. I made the following mistakes:

• The WebUI of my OPNsense boxes are on a non-standard port (because I am using the standard 80 and 443 ports for Nginx). Under System > High Availability > Settings, I had to put the full URL into the "Synchronize Config to IP" field on the primary (i.e. https://<Backup pfSync IP Address>:4444)
• On the backup box, under System > Settings > Administration, I had to add the pfSync Interface into the "Listen Interfaces" selection (it was previously only set to "LAN").

Hope this helps someone in the future.