Server shuts down whenever friend tries to join

We've been playing DST with my friend for a while now, and it was fune until today, when I tried to make a new world. I could join without problems, but whenever my friend joins the server, it suddenly shuts down, and I don't understand why. It worked before, and nothing changed since then.
The server log says at the bottom, that it received a shutdown signal, and it just shuts down by itself. I don't know what could cause this problem. Can you look into it?