Windows® 8.1 is a newer operating system and it works better with UEFI instead of legacy. Also USB 3.0 is a feature of UEFI. I suggest you to reset BIOS settings to default. Get into the BIOS, press F9 then, F10.
Now, plug the bootable USB drive with Windows® 8.1 to the NUC, turn the kit on and keep pressing F10, select UEFI – USB bootloader, format the SSD or HDD then, continue the operating system installation.
Thank you mikec_intel for the answer. For now I disabled the "Failsafe Watchdog" to avoid the boot error message. All in all I have to say it feels like a stable and good system now. So because of different problems I already installed windows three times and updated BIOS from 024 to 036 since I have got the NUC for two weeks ago and I think I wait with installing everything a fourth time. But in the next weeks I will do this.