org.json.JSONException: End of input at character 0 of

For some weird reason if you use .string() >1 time, response turns out to be empty. Try to use it only once when parsing Retrofit response.

Someone, who knows why it happens like this, please improve my answer.

Answer : Response object should of Generic type - ResponseBody.
See below Correct code for reference.
Now response.body() method will return object ResponseBody i.e.

ResponseBody rb = response.body(); 
rb.string();

Here ResponseBody have method string() which returns String object but internally string() method calls Util.closeQuietly(source); which makes response empty once method string() gets called.

Just remove Log.d(TAG, response.body().string()); and follow below code.

Reference - okhttp3.ResponseBody.java

error : org.json.JSONException: End of input at character 0

Correct code :

@Override
public void onResponse(Call call, Response<ResponseBody> response) throws IOException {
   if (response.isSuccessful()) {

      String remoteResponse=response.body().string();
      Log.d(TAG, remoteResponse);
      try {
          JSONObject forecast = new JSONObject(remoteResponse);
      } catch (JSONException e) { 
          e.printStackTrace();
      }
   }
}