I've been running this server for a while, and decided to give the snapshots a whirl again...
Whenever a player logs in, they connect properly but are then disconnected while downloading terrain. I've attached a text file with the relevant parts of the server log.
I started the server in 12w13b, and the error happened.
Then, upon hearing of a more recent version I tried it and it still happened.
Due to it being a log-in crash, I've removed the gamemode field for its irrelevancy.
Duplicate of MC-15058.