Quoting workshop 25:"Start the server first: check both Client and Server, then hit Ok and start the script as usual."
and that is exactly what I did, I started the first engine as a client and a server. The second engine only as a client, but as I said that didn't work.
I also tried Quadraxas suggestion: first check only the server and than only client, but I get exactly the same errormessage when running the server.