yes, strange, isn´t it?
As I don´t have a screen installed there, I only run the computer with an RDP-session. But even in the RDP session I am able to start the program myself directly and everything works correctly. The problem occus only, when started by the tas scheduler. By the way: meantime I tested another task scheduler by a third party, and effect is the same: blank screen
Interesting is also, what I now found out: When last time I accessed the RDP session by teamviewer (third party remote control software) and not by windows remote desktop, later screenshot captor is being started by task scheduler and works!
Thanks for your thoughts,