!!! NOW IT IS SOLVED !!!
in the described folder were my old key. this old key did not unlock vrayscatter cause it was from an older installation of my windows system. (read above)
the new key(s) could not be applied cause the old key in this folder were write protected. therefore always "Can't apply Key File" appears with "VRayScatterRegistration_2.exe". to avoid this for other users "VRayScatterRegistration_2.exe" should copy the key file and change the write protection if the source key is write protected.
nice day! thanks for help again!