What is status code 9 in Kong

I have run Kong 1.4 in Kubernetes with Postgresql. Both Kong and DB are in K8S while Kong has 3 replicas and Postgres is Statefulset with 1 Master and 1 Slave.

I suddenly found the error from Kong log in Kibana with status code 9 and I have no idea what it is.
One thing in common is the request was held in Kong for 2 seconds and then release as an error.

Anyone have an idea of what is the root cause of this.