As far as I can tell, what's going on may be that I_GetNodeAddress(node) (which gets the IP address of someone in the game) can possibly return NULL (for some reason), and then the game tries to copy this non-existent string using strcpy to another place when a new player is joining.
Thanks to Spectrum and Inazuma for helping me figure this out and putting up with my requests for logs and spamming debug messages in my special exes. =>
(also this needs testing I guess but there we are)