Boot Camp Error rMBP "15 2015 newest Setup error without message

Hi,

i can't install the bootcamp drivers (5.1.5917) on my new MacBook Pro with BootCamp and Windows 8.1.

While installing the bootcamp drivers from the usb drive (the drivers the bootcamp assistant adds to the usb drive) - a message appears shortly after i agree the terms. A message without any information. After i click ok the setup stops and asks to restart the macbook.

No software get's installed. I.e. I cannot use the keys for changing my display brightness or mute the sound. Even with the fn key.

i hope someone can help me!

cheers,

micha