This is exactly what happened to me! Now i am going to try that m.2 trick too. Will keep you guys updated.
Update: Finally made it work. Not by using m.2. Just switch the pci-e slot and finally find one combination works. What a day...
Do you recall what you changed? Did you just swap cards in random slots until it worked? Were you installing drivers? And did you get 5 or 6 working?
Nothing really changed. I just swap the original slots for these cards. Seemed to me that Windows remembers things and it is determined sometimes. You need to force it to re-remember. Now I got 6 working and it is stable.
Update: as I posted earlier, I couldn't get more than 5 working in my asrock. When I put the 6th in it would reboot. I thought on a whim I'd try a different card. So the 1060 wouldn't work but for some reason a 580 magically does.