When configuring some Oracle REST web services through Oracle APEX, I got confused, so I thought to quickly write down, so I don't forget for the future.
Although I use many ORDS web services in several schemas, for a new schema I got following error: "The database user for the connection pool named |apex|rt|, is not authorized to proxy to the schema named ."
I was confused, as I enabled the schema to be REST enabled.
It was only once I gave following permissions it worked:
alter user {USER}
There you go, it might help you too :)