Value Access of type java.lang.String cannot be converted to JSONObject

I have a problem with my Web Service, I am trying to obtain my values ​​that are stored in a database, but when I consult the web services from the browser it gives me the correct information which makes me think that the WbService already works, the problem is that now I want to consult it from an android application but it gives me this error:

Value Access of type java.lang.String cannot be converted to JSONObject

I don't understand this error, my Web Service is like this:

<?PHP
require( dirname(__FILE__) . '/wp-load.php' );

$hostname_localhost = "localhost";
$database_localhost = "prueba";
$username_localhost = "root";
$password_localhost = "";

$json=array();

    if(isset($_GET["name_usu"])){
        $nombre_usuario = $_GET["name_usu"];
        $password = $_GET["password"];

        include 'PasswordHash.php';

        echo $pass;

        $conexion = mysqli_connect($hostname_localhost, $username_localhost, $password_localhost, $database_localhost);
        mysqli_set_charset($conexion, "utf8");
        $consulta="SELECT ID, user, pass, nickname, email, url, fecha_registro, name_mostrar FROM usuarios WHERE user = '{$name_usu}'";
        $resultado=mysqli_query($conexion,$consulta);

        if($registro=mysqli_fetch_array($resultado)){



        $wp_hasher = new PasswordHash(8, true);   // 16 digit hashing password
        $pass = $wp_hasher->HashPassword($_POST['password']); //$posted['password'] is your password

        if($wp_hasher->CheckPassword( $password, $registro[2] )){
            echo "Acceso correcto";
            //Si todo funciona mandaremos la informacion a este array
            $json['sesion'][]=$registro;

        }else{
            echo "El usuario es incorrecto";
        }

        }else{
            $resultar["ID"]='No registrado';
            $resultar["user"]='No registrado';
            $resultar["pass"]='No registrado';
            $resultar["nickname"]='No registrado';
            $resultar["email"]='No registrado';
            $resultar["url"]='No registrado';
            $resultar["fecha_registro"]='No registrado';
            $resultar["name_mostrar"]='No registrado';
            $json['sesion'][]=$resultar;
        }

        mysqli_close($conexion);
        echo json_encode($json);

    }else{
        $resultar["success"]=0;
        $resultar["message"]='Ws no Retorna';
        $json['sesion'][]=$resultar;
        echo json_encode($json);
    }
?>

Successful access{"session":[{"0":"1757","ID":"1757","1":"testdb","user":"testdb","2":"$P$BhbEVMLV6onULEfYLG3dsF5xuv9t9j0 ","pass":"$P$BhbEVMLV6onULEfYLG3dsF5xuv9t9j0","3":"testdb","nickname":"testdb","4":"[email protected]","email":"dbtest@outlook. com","5":"","url":"","6":"2019-02-20 15:35:53","date_registration":"2019-02-20 15:35:53" ,"7":"enrique espinosa","display_name":"enrique espinosa"}]}

And in my application I have this in the OnResponde method and in the onErrorResponse:

@Override
    public void onResponse(JSONObject response) {

        JSONArray json = response.optJSONArray("sesion");
        JSONObject jsonObject = null;

        try{
            jsonObject = json.getJSONObject(0);
            ID = (jsonObject.getInt("ID"));
            user = (jsonObject.getString("user"));
            pass = (jsonObject.getString("pass"));
            //claveBD = (jsonObject.getString("clave"));
            nickname = (jsonObject.getString("nickname"));
            email = (jsonObject.getString("email"));
            url = (jsonObject.getString("url"));
            fecha_Registro = (jsonObject.getDouble("fecha_registro"));
            nombre_mostrar = (jsonObject.getString("nombre_mostrar"));
            //(jsonObject.getString("recordado"));

        }catch (JSONException e){
            e.printStackTrace();
        }

        if (nombreBD.equals("No registrado") || passwordBD.equals("No registrado")){
            Toast.makeText(getApplicationContext(), "Los datos son incorrectos", Toast.LENGTH_LONG).show();
        }else if(passwordBD.equals(passwordCaja)){

            registrarUsuario(nombreBD, passwordBD,"master",rolBD, empresaBD, box);

        }else {
            Toast.makeText(getApplicationContext(), "La contraseña es incorrecta", Toast.LENGTH_LONG).show();
        }
    }

    @Override
    public void onErrorResponse(VolleyError error) {
        Toast.makeText(getApplicationContext(), "Error al conectar al servidor: "+error, Toast.LENGTH_LONG).show();
        Log.i("servidor","Error de conexion: "+error);
        error.printStackTrace();
    }