There are no firewall rules preventing traffic on the direct ethernet link between the two firewalls.
Thanks for the suggestion. Yes, there is. This is a previously working config that appears to have stopped working at some point. I did have to reinstall the primary server at one point and did so using the USB stick config transfer method. No passwords have changed. The problem is that the diagnostic message I'm getting is so non-specific as to leave me lost as to how to even investigate what's not working.
root@<host>:~ # /usr/local/etc/rc.filter_synchronizesend >>>Host: 192.168.66.4User-Agent: XML_RPCContent-Type: text/xmlContent-Length: 117Authorization: Basic cm9vdDpQaWJqSXBzSUxwVEFmNHlZOTZ4Uw==<?xml version="1.0"?><methodCall><methodName>opnsense.firmware_version</methodName><params></params></methodCall>received >>>error >>>fetch error. remote host down?root@fenchurch:~ # send >>>Missing name for redirect.<methodName>opnsense.firmware_version</methodName><params></params></methodCall>received >>>error >>>fetch error. remote host down?