In the command to start the Natural RPC Server you specified
SRVNODE=ETB001
This Node is passed to the Broker stub as the Broker ID and the Broker stub must be able to get the tcp/ip address from the Broker ID (using the TCP/IP function gethostbyname). The Broker ID ETB001 may not be known by TCP/IP and this could be the cause of the issue.
Instead of ETB001 you can try to use the machine name where the Broker is running (or localhost if the Broker is running on the same machine). If the Broker does not use the default port (1971) you also have to specify the port.
Please change the SRVNODE from ETB001 to :
Example
SRVNODE=localhost:1971
If this does not help, please open a support request and provide a Natural RPC Server Trace (with trace level 2) and a EntireX Stublog of Level 4.