When I joined my Vanilla 14w28a server, everthing was okay. When I walked: server crashed.

To test if it only crashed on incomming packets, I did a little experiment.
When I joined, I went to the console and run the command: "/say Hello"

In the client, I could see the message, so the server doesn't have any problems with the outgoing packets. However, when I tried to chat something back, the server immediately crashed.

A crash report is unnessecairy, because it will happen to to, too.