_monad

Thank you so much for your help! I've found the solution (though I don't know why it is the solution).
 
On a hunch I tried all the different usb ports of the motherboard and there are two, from which unraid boots fine. I have no idea why those two (especially, since these are different ports than the ones I used in the past for this mb and unraid), but right now I'm just happy that it works.