Serviço Rest está fora?

Executei o programa do professor disponibilizado mas ainda não encontrei o erro.

Erro que tenho é esse:

11-01 03:55:41.076 2403-3039/csc.com.agenda W/System.err: java.io.FileNotFoundException: https://www.caelum.com.br/mobile
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:238)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.getInputStream(DelegatingHttpsURLConnection.java:210)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at csc.com.agenda.WebClient.post(WebClient.java:55)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at csc.com.agenda.EnviaAlunoTask2.doInBackground(EnviaAlunoTask2.java:41)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at csc.com.agenda.EnviaAlunoTask2.doInBackground(EnviaAlunoTask2.java:18)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at android.os.AsyncTask$2.call(AsyncTask.java:295)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at java.util.concurrent.FutureTask.run(FutureTask.java:237)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
11-01 03:55:41.430 2403-3039/csc.com.agenda W/System.err:     at java.lang.Thread.run(Thread.java:818)

Problema resolvido, o erro estava na conversão para JSON, estava usando "alunos" ao invés de aluno.