When I created my array for the first time, I pre-cleared all disks and it was all fine. I then added them to my array. It turned out, one disk has failed (no temp, lots of read/write errors, it turned out I didn't put it strong enough in the hot swap bay, but that's another story).
As this drive was faulty, I put it out and my array config got rendered invalid, thus I recreated my unraid usb stick and assigned all the drives from scratch. The creation of the array went fine without clearing by unraid itself, even though preclear_disk.sh -t /dev/sdX on all the array disks showed clearly, that the preclear signature is gone. Then I used the Format button to format all the drives.
Let me know if what I did was correct or if I should have done another preclear-cycle (I already did 3-4, depending on the drive).
Hopefully my parity is valid.