keeps kicking all players from multiplayer server with error connection lost internal exception

I have a small multiplayer server running 17w15a and all players are being continually kicked with the msg. Connection Lost Internal Exception:io.netty.handler.codec.DecoderException:java.lang.NullPointerException.
One person says it happens when he runs and jumps. It seems to be arbitrary with me. I was able to play for about 15 min before it started doing this now I can only stay on for about a minute before it kicks me.