Thanks for the hint, I actually did not setup Little Snitch correctly so it kept closing Factorio's connexion to localhost.

When I launch a LAN multiplayer game, the game crashes after ~2 seconds.
I'm on macOS Big Sur (RC 2), I have no mod, and the log file seems to show no error, and macOS crash reports does not notice Factorio's crash.
I have no issue when I host an online multiplayer game neither on solo.
(I don't ...