Attached touch screen ends up with a white screen

Ladies and gents,

a few month ago I my perfect working 3.5" touch display faild to display my TouchUI.
The screen was always white from now on.

Now I startet a complete new installation on my Pi3 and octopi is running well.
The initial installation of the screen worked out well as well.

Unfortunately a display waiting in promt is almost useless.
I installed TouchUI again and the screen is white again!?

Any Idea which direction to go?
Any clue?

I am not a pro but not a noob.
Hopefully someone can push me to a solution.

Thanks in advance
Cappy