I am trying to send some data to the server using the Volley library.
private void registerUser(final String email, final String username,
First of all you are not sending json to your server. You are sending parametrized url with post method and getting json text as response.
I think the problem is the response from the server which is supposedly json. But from your log, this text "---- YOUR DATA ----"
which you get from Log.d(TAG, "Register Response: " + response.toString());
is not at all json formatted . So you need to modify your server to respond in correct json format.