java.lang.NullPointerException, cannot connect to old world

I am unable to play on my server, because I get an error ("Connection Lost: End of stream" in client)( http://pastebin.com/0amxGU6u in server) whenever I try to connect to my survival server after upgrading from 1.5.2 to 13w18c.

Also, I get "[SEVERE] Reached end of stream" in the server console every time my client tries to get the ping and connection (the refresh button in the server manager).