As you know, I was not able to repeat the daydream black screen.
I have turned on the screen lock and this made the trick.
I can easily see the problem now.
Thank you so much for directing me.
But I have bad news.
I don't think there is anything I can do to solve the problem.
YoWindow is displaying the graphics with Open GL technology.
Every 3D game or wallpaper uses Open GL.
I have tried other 3D daydreams (screen-savers).
All of them turn black as soon as screen is locked.
It seems that Android is blocking Open GL (3D) visualisation by some reason when screen lock is engaged.
If you know any 3D daydream or screen-saver that survives screen lock, please let me know.
We will try to figure out how does it work.