The start of an SAP-agent fails with the error message:
U02004254 Connection Object 'CONN.ERE_1_NEU' is not a valid 'ABAP' connection, entry 'rfcHostname' is missing.
The internal parameter rfcHostname refers to the parameter Application Server in the Connection Object when Custom Application Server is used for RFC Connection Type.
In this case, Group/Server Selection was used as RFC Connection Type.
I.e. Application Server was not used and still, the error message says the parameter is missing.