ok before you send me to the glue factory- I had actually answered my own question in an earlier thread, but for those that strike this issue in future the full answer is-
When conecting fleetctl to Security Onion, leave the port number out of the ‘config set’ command
ie. don’t use
fleetctl config set --address https://so.mydomain.com:8090 --url-prefix /fleet
fleetctl config set --address https://so.mydomain.com/ --url-prefix /fleet
Fine, yes I was pretty dumb.