So you should first run "killall pd" (and even killall -9 pd when it's stuck), before running you command.
Thanks! You are correct that a pd process with the netreceive is still running. (fun fact, for me at least: When running it from a new terminal, the new netsend started printing on the previous terminal)
Now I tried it on a different mac. I was on macOS 10.9, now on 10.11.6 (same pd)
On first run, connection is established and patch works as expected, which is puzzling.
On second run (after effectively running killall pd, or killall -9 pd):
connecting stream socket: Connection refused (61)
Fede, can you confirm that no other Pd instance is running before you start your second instance?
I remember issues with the operating system leaving a port open and thus reserving it for a longer while than the application (Pd in this case) requests it. You could try to use a patch with a different port number when you start it the second time and see if that works.