You should set that IP there that is used on the servers to communicate with each other, and that's normally the internal one unless you blocked access between the systems with a firewall or similar. Maybe you don't have the hostnames of all systems with their internal IP's in the hosts files so the database server can't authenticate the user from web server?
That is possible, I have done a lot of this work around my full time job so late nights making changed could have let in a misconfiguration on my part. I will double check each of the servers. As i said, its working now as I added the hostname user, ive also reverse proxied my setup so now the users I added are probably defunct. I will have to do some thorough checking to find out which users are being used under which configuration. Back to my aside, i search issues, I noted your feature from some years back, nginx reverse proxy, ive commented on it as that was what I was going to request.
