[backwards]

I don't see why having 1 thread would be an issue other then lag of course. It shouldn't cause the server to crash unless they've added something recently to prevent multiple servers from running on the same thread. Unless you have a type of crash log or can get the output from the console I'm not sure how else i can help. It's possible you have a plugin causing conflicts as well, if they connect to a remote server or something like that for instance and are unstable when ones already connected.