According to my experiments, when a world is opened to LAN, the game sends multicast pings to other machines on UDP port 4445, with the string "[MOTD]<username> - <world name>[/MOTD][AD]<port number>[/AD]". However, if the world name includes the string "[/MOTD]", the game does not change that string in the UDP packet, so the packet might end up looking like "[MOTD]person - [/MOTD][/MOTD][AD]7223[/AD]", which is malformed, so it is impossible to open that world to LAN.