O json não aparece no Toast.

Quando clico no item para enviar os dados o Toast vem vazio. Já verifiquei se minha lista de alunos é convertida em Json e ela é sim. Percebi que uma exceção é lançado, será que é por conta disso? Segue o printstacktrace:

W/System.err: java.io.FileNotFoundException: https://www.caelum.com.br/mobile
W/System.err:     at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:251)
W/System.err:     at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.getInputStream(DelegatingHttpsURLConnection.java:210)
W/System.err:     at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.getInputStream(Unknown Source:0)
W/System.err:     at br.com.atech.agenda.WebClient.post(WebClient.java:32)
W/System.err:     at br.com.atech.agenda.SendStudentsTask.doInBackground(SendStudentsTask.java:33)
W/System.err:     at br.com.atech.agenda.SendStudentsTask.doInBackground(SendStudentsTask.java:17)
W/System.err:     at android.os.AsyncTask$2.call(AsyncTask.java:333)
W/System.err:     at java.util.concurrent.FutureTask.run(FutureTask.java:266)
W/System.err:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:245)
W/System.err:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
W/System.err:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
W/System.err:     at java.lang.Thread.run(Thread.java:764)

Meu WebClient.class:

public class WebClient {
    public String post(String json){
        try {
            URL url = new URL("https://www.caelum.com.br/mobile");

            HttpURLConnection connection = (HttpURLConnection) url.openConnection();

            connection.setRequestProperty("Accept", "application/json");
            connection.setRequestProperty("Content-type", "application/json");

            connection.setDoInput(true);
            connection.setDoOutput(true); // aqui estamos fazendo um post, por isso tem OutPut

            PrintStream output = new PrintStream(connection.getOutputStream());
            output.println(json);

            connection.connect();

            String response = new Scanner(connection.getInputStream()).next();
            return response;
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
}

Minha classe Task:

public class SendStudentsTask extends AsyncTask<Object, Object, String> {
    private Context context;

    public SendStudentsTask(Context context) {
        this.context = context;
    }

    @Override
    protected String doInBackground(Object... objects) { //varials?

        StudentDAO dao = new StudentDAO(context);
        List<Student> students = dao.buscaAlunos();
        dao.close();
        StudentConverter converter = new StudentConverter();
        String json = converter.convertToJson(students);
        WebClient client = new WebClient();
        String response = client.post(json);
        return response;
    }

    @Override
    protected void onPostExecute(String response) {
        Toast.makeText(context, response, Toast.LENGTH_LONG).show();
    }
}