Auth/Az web service is working as expected when client connects directly to Access Gateway, but is failing with an error when going through the external load balancer. The problem is not occurring in the pre-prod environment in which the requests are going through the internal load balancer.
The external load balancer had different, more secure settings than the internal load balancer. Load balancers and/or proxies need to be transparent to the hosts behind them. In this instance, the external load balancer was updating the HTTP_HOST header with the internal host name rather than preserve the hostname used by the client. This caused the request to be handled by the Default virtual host rather than the intended one for Auth/Az web service and thus the error.
Release : 12.8.03
Component : ACCESS GATEWAY
Update external load balancer configuration so that it is transparent to the hosts behind it.