Okay, I wasn't sure what was causing the error I kept getting, but now I'm 100% sure it's your mod.
Your code works fine in SP, but in MP it randomly creates the same error over and over. It happens only when generating chunks in Slaanhatten, and only in MP. It stops the server from generating chunks and desynchronizes all clients, making reconnecting impossible. This behavior is slightly different in case of a dedicted server and a privately hosted MP game, but the cause is exactly the same.
I've found there was a discussion about a similiar issue HERE
So for now the only way to get Slaanhatten to an MP game is generating it with your code and then removing all your code completely form the rwgmixer. I believe it's some minor mistake in the code, but it's quite difficult for me to track it down, as I don't know the city hub creation mechanics well enough...