Black Screen on WP8 Device(Nokia 520)

Hello,

i am new here and i have build my first Xamarin.Form App. It a simple Countdown(to my next holiday).
Target are WP and Android.
I code it and test it again Nexus 7 and my Nokia 930. Both are running.
So i upload it to the Store and puplish the Link to my Wife and a friend.
My friend have a Nokia 620 or Nokia 720 and the app work fine.
My Wife have a Nokia 520 and there i have only a black screen.
I plug the Nokia on my dev pc and try to debug that. I have the black screen too,
but i can see that the app work in the backgroud like on my 930.
I see that the timer run fine and the Countdown work, but no Display.

I have on page with AbsoluteLayout plus some Labels.
Something like that(i have only a older Version here):
` this.TitleLabel=new Label {Text = "Crazy Trip '15 in"};
this.LayoutType = new AbsoluteLayout
{
BackgroundColor = Color.Blue.WithLuminosity(0.9),
VerticalOptions = LayoutOptions.FillAndExpand
};

        AbsoluteLayout.SetLayoutFlags(this.TitleLabel,
           AbsoluteLayoutFlags.PositionProportional);
        AbsoluteLayout.SetLayoutBounds(this.TitleLabel,
            new Rectangle(0.5,
                0.4, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize));

        this.LayoutType.Children.Add(this.TitleLabel);

        this.Content = this.LayoutType;`

I have second label where i show the timespan.

Any ideas, why the Display on the Nokia 520 show a blank black screen?
(The emulator, which are near to Nokia 520 run fine).

Thank and BR
Tobias