To clarify this issue:
I played the this game last week on this PC. Yesterday the game made an update (AND JUST THE GAME - between playing I didnot restart my PC), after which it won't start anymore with an windows - error msg, that the game is not compatible with Windows 8.1 anymore.
I tested the game on my Laptop running with Windows 10 and it works fine.
Both PC / LAPTOP Systems use the THE SAME ANTIVIRUS Programm. Even after disabling the ANTIVIR the game's not starting on the Win 8.1 PC.
So I tend to believe there's an issue with the last game update and the windows 8.1 compatibility but nvm ....