SSLHandshakeException

Estou recebendo a seguinte exceção:

W/System.err: javax.net.ssl.SSLHandshakeException: SSL handshake aborted: ssl=0xa2338400: I/O error during system call, Connection reset by peer
W/System.err:     at com.android.org.conscrypt.NativeCrypto.SSL_do_handshake(Native Method)
        at com.android.org.conscrypt.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:318)
        at com.android.okhttp.Connection.upgradeToTls(Connection.java:201)
        at com.android.okhttp.Connection.connect(Connection.java:155)
        at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:276)
        at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:211)
        at com.android.okhttp.internal.http.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:382)
        at com.android.okhttp.internal.http.HttpURLConnectionImpl.connect(HttpURLConnectionImpl.java:106)
W/System.err:     at com.android.okhttp.internal.http.HttpURLConnectionImpl.getOutputStream(HttpURLConnectionImpl.java:217)
        at com.android.okhttp.internal.http.DelegatingHttpsURLConnection.getOutputStream(DelegatingHttpsURLConnection.java:218)
        at com.android.okhttp.internal.http.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java:25)
        at alura.com.agenda2.WebClient.post(WebClient.java:26)
        at alura.com.agenda2.EnviaAlunosTask.doInBackground(EnviaAlunosTask.java:35)
        at alura.com.agenda2.EnviaAlunosTask.doInBackground(EnviaAlunosTask.java:18)
W/System.err:     at android.os.AsyncTask$2.call(AsyncTask.java:292)
        at java.util.concurrent.FutureTask.run(FutureTask.java:237)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:818)

Código WebClient.java:

import java.io.IOException;
import java.io.PrintStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Scanner;

import javax.net.ssl.HttpsURLConnection;

public class WebClient {
    public String post(String json){
        try {
            URL url = new URL("https://www.caelum.com.br/mobile");
            HttpsURLConnection connection = (HttpsURLConnection) url.openConnection();
            connection.setRequestProperty("Content-type","application/json");
            connection.setRequestProperty("Accept","application/json");

            connection.setDoOutput(true);
            connection.setDoInput(true);

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

            connection.connect();

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

        return null;
    }
}

Código AlunoConverter.java:

import org.json.JSONException;
import org.json.JSONStringer;

import java.util.List;

import alura.com.agenda2.modelo.Aluno;

public class AlunoConverter {
    public String converteParaJSON(List<Aluno> alunos) {
        JSONStringer js = new JSONStringer();
        try {
            js.object().key("list").array().object().key("aluno").array();
            for (Aluno aluno: alunos) {
                js.object().key("nome").value(aluno.getNome()).key("nota").value(aluno.getNota());
                js.endObject();
            }
            js.endArray().endObject().endArray().endObject();
        } catch (JSONException e) {
            e.printStackTrace();
        }
        return js.toString();
    }
}

Em EnviaAlunosTask.java:

   @Override
    protected String doInBackground(Void... objects) {
        AlunoDAO dao = new AlunoDAO(context);
        List<Aluno> alunos = dao.buscaAlunos();
        dao.close();
        AlunoConverter conversor = new AlunoConverter();
        String json = conversor.converteParaJSON(alunos);
        WebClient webClient = new WebClient();
        String resposta = webClient.post(json);
        return resposta;
    }

Será que o WebService ainda está no ar?