Android - Error de permisos en juego 2D LibGdx

com.badlogic.gdx.utils.GdxRuntimeException: Error writing file: highscores.dat (External)

at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:353)

at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339)

at com.mygdx.game.game.HighScores.save(HighScores.java:52)

at com.mygdx.game.game.HighScores.load(HighScores.java:15)

at com.mygdx.game.pantallas.PantallaFinJuego.show(PantallaFinJuego.java:93)

at com.badlogic.gdx.Game.setScreen(Game.java:61)

at com.mygdx.game.pantallas.PantallaJuego$5.run(PantallaJuego.java:368)

android:versionCode="1"

android:versionName="1.0" >

<uses-sdk android:minSdkVersion="8" android:targetSdkVersion="23" />

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

<application

    android:allowBackup="true"

public class HighScores {

public static String[] highscores = { "0", "0", "0" };

public static String archivoHighscores = "highscores.dat";

public static void load() {

    FileHandle arquivo = Gdx.files.external("highscores.dat");

    String linea;

    if (!arquivo.exists())

        HighScores.save();

    linea=arquivo.readString();

    highscores=linea.split(",");

    // ---- CÓDIGO ----

}

public static void engadirPuntuacion(int puntuacion) {

    boolean encontrado = false;

    int i = 0;

    load();

    while ((i < HighScores.highscores.length) && (!encontrado)) {