Servers in 15w31c (for me) Cant join them or then i get kicked

When I try to join my server that me and my dad set to 15w31c I cant join because it has some code text and at the top it says conntection lost and heres the code thing of text:

Internal Exception: io.netty.handle.codec.DecoderException:com.google.gson.JsonSyntaxException:com.google.stream.MalformedJsonExceptionJsonReader.setLenient(true) to accept malformed JSON at line 1 column 1