@braunerroei Then your frontend config looks like this?
196c1e01-1e74-49f5-87c8-4d22eb7bf590-image.png
That's the SSL Offloading I was talking about. If you don't check that box, then pfSense won't negotiate SSL. I was worried that you might be processing unencrypted.
In any event, I resolved my 503 problems. I'm not using the default port 443 for this new connection. Therefore, the value of the "Host Matches" ACL entry needed to be my.host.com:6407. I had used my.host.com with no port.
I had assumed that HAProxy would tack the port number on to the value because the port number is specified in the external address table. I see now it can't do that. The External Address table may contain multiple entries. It follows that the ACL match routine has no way to know your intentions unless you specify the port number in the ACL.
Thanks for the help. Your answers got me questioning my own configuration which turned out to be in error.