I created a server with my old world on version 15w31a but failed to join and this shows up:
Internal Exception: io.netty.handler.codec.DecoderException: com.google.gson.JsonSyntaxException: com.google.gson.stream.Malformed.JsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1

The world had no problem when being opened in server of version 1.8 onwards and was able to join.

Steps to reproduce:
1)Put the world file in server folder.
2)Open the server with that world file.

*Note: The region file in the world folder is too huge so i split it into 4.
I am not sure that this issue could be open to public because it contains private informations.