Access Gateway servers are returning HTTP 500 for requests that are received which lacks the Host header.
Release : 12.8.06
This is by design that Access Gateway will throw HTTP 500 when it receives a request that lacks HOST header.
Access Gateway requires the HOST header to identify which virtual server need to process this request and this is by comparing the host header.
Upon listing all the HTTP 500 errors, it had a patter of specific time interval suggesting this was a health check request.
It is the load balancer that stands in front of the Access Gateway checking the status.
Load Balancer need to be updated to send a request with the host header to avoid unnecessary HTTP 500.