java.net.UnknownHostException

Here is code 

String urlToken = "https://api.dropboxapi.com/oauth2/token";
String charset = "UTF-8";  // Or in Java 7 and later, use the constant: java.nio.charset.StandardCharsets.UTF_8.name()
String query = String.format("grant_type=authorization_code&%s&code=%s&client_id=%s&client_secret=%s",
        "redirect_uri=https://192.168.137.129:8080/0/rest/cloudAdded?provider=dropbox",
        URLEncoder.encode(authCode, charset),
        URLEncoder.encode(APP_KEY, charset),
        URLEncoder.encode(APP_SECRET, charset));
Log.d(LOG_TAG, "getInputStreamAsString()");
String inputStreamAsString = AccountsManager.getInputStreamAsString(urlToken, charset, query);

String result = "";
try{
    URL url = new URL(urlStr);
    HttpsURLConnection conn = (HttpsURLConnection) url.openConnection();

    // Create the SSL connection
/*SSLContext sc;
sc = SSLContext.getInstance("TLS");
sc.init(null, null, new java.security.SecureRandom());
conn.setSSLSocketFactory(sc.getSocketFactory());*/

    // set Timeout and method
    conn.setReadTimeout(15000);
    conn.setConnectTimeout(15000);
    conn.setUseCaches(false);
    conn.setDoInput(true);
    conn.setDoOutput(true); // Triggers POST.

    conn.setRequestProperty("Content-Language", "en-US");
    conn.setRequestProperty("Accept-Charset", charset);
    conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
    // conn.setRequestProperty("Content-Length", "" + Integer.toString(_charset.getBytes().length));

    OutputStream os       = conn.getOutputStream();
    os.write(query.getBytes(charset));
    os.close();

    conn.connect();

/*
String err = new String();
InputStream errStream = conn.getErrorStream();
BufferedReader errReader = new BufferedReader(new InputStreamReader(errStream));
String eLine;
while ((eLine = errReader.readLine()) != null) {
    err += eLine;
}
Log.e(LOG_TAG, "ERROR : " + err);
*/

    Log.d("", "getInputStreamAsString() : responseCode: " + conn.getResponseCode() +
            "response message : " + conn.getResponseMessage());

    //read the inputStream into a string
    result = new String();
    InputStream is = conn.getInputStream();
    BufferedReader in = new BufferedReader(new InputStreamReader(is));
    String inputLine;
    while ((inputLine = in.readLine()) != null) {
        result += inputLine;
    }
} catch (Exception e) {
    Log.w("", "Exception thrown " + e.getMessage());
    throw new UnsupportedOperationException("TODO : handle this");
}

Most of the time code is working fine. But Some time I am getting error.