Problem Solved itself. -.-;
I dont know why, but after starting the Activation Tool directly from the Directory the game launched.
Had to terminate the Activation.exe via Taskmanager, then started it directly from the directory. Worked even without asking for the key.
Maybe the Activation.exe causes the problems, but not sure about that.
So the only problem seems like now the updating...
System: WIN7 PRO 64bit, Icore7 860, GeForce 470GTX, 8 GB RAM