"Incomplete set of tags receive from server. Please contact server operator."

I am currently trying to set up a server a group of friends can play on, however whenever one of us tries to join, this error message comes up: "Incomplete set of tags receive from server. Please contact server operator." I don't really know if there is something I am doing wrong, it loads the terrain the world is generated on the logs, but whenever someone joins it disconnects us and comes up with the error message.