I started from scratch once again.
- Partitioned the SSD with gparted (GPT) with two partitions, first one NTFS for Windows, second one HFS+ for Mac
- went through the Mac installation since I thought Yosemite would make its own EFI partition which would also work for Windows since it doesn't work the other way around, had to erase the Mac partition before installing so that it would really be Mac OS Extended Journaled (activating Journaling didn't cut it)
- after the installation I rebooted with the Usb and installed Clover and some kexts
- then I installed Windows 8.1, it went through until it came to the Restart Screen, I pressed Enter to restart
And since then I couldn't get the Laptop to boot again or to go into the Bios for that matter. It is just once again a black lit up screen and no matter what keys I press, nothing happens.
I already removed the battery and pressed the power button for 30 seconds. Swapped the slot of the SSD - nothing.
Maybe someone here can make sense of this interesting development.