I have this simple service on my server at :

router.route('/hooray/')
    .get(function (req, res) {
        res.status(200).send({ result: true, message: "hooray" })
    })

It does work from Postman, but from my Android application I unable to reach that url, and I don't know why.

That's the (simplified) code:

Service's Interface

public interface HoorayAPI {
    @GET("hooray/")
    Observable<HoorayResponse> hooray();
}

My custom Response class: HoorayResponse

public class HoorayResponse{
    @SerializedName("result")
    @Expose
    private boolean result;

    @SerializedName("message")
    @Expose
    private String message;

    public HoorayResponse(boolean result, String message) {
        this.result = result;
        this.message = message;
    }

    // Getter and setter ...
}