Your blogpost was very interesting, thank you for that! Is it best practice in every big company to just use proxy-users or hand over the login for a certain schema, when a not a schema owner user tries to access the schema and/or change/add/delete objects in it?

I actually figured that out while you typed it... Well I guess sometimes one just needs a bit more time for themselves... Anyway the Solution was a firewall problem. For default settings on port 1521 you have to create a new rule allowing incoming traffic on port 1521 from anywhere. You can either check the connection by using the client OR try to use lsnrctl status on the client. It will deny access due to insufficient privileges. This is because no client should be able to start, stop or status the listener for an entire oracle network...