M.2 -> PCIe adapter didn't help. The card is not detected by windows when connected to M2_2, and goes crazy when connected to M2_1 (just the same as to PCIE4).

Tried ubuntu 17.10 and arch. Kernel panic on boot.

Also I don't recommend updating bios to P4.40. It doesn't boot even with 5 cards. Didn't try less cards.

It is stable on P3.20 with 5 cards and win10 though. So I guess I have to leave it at that.

No.

Risers are needed either for pciex16 -> pciex1 transition or if directly installed card blocks another slot.

If you can install 3 cards in 3 slots directly, do so. It's probably even better.

Missed this one. Will try m2 > pcie adapter.

I am trying to build a rig of 6 RX570 on ASRock B350 Pro4 (with Ryzen 3 1200).

Ubuntu 16.04.3 doesn't boot when even just one card is connected with riser. Fails with kernel panic. Both live-usb and installed without connected riser.

On Windows 10, 5 cards are working good.
But the 6th card (that is connected to second PCIe x16 with riser) isn't detected properly most of the time (like 4 in 5 boots). And even when it is detected, after some time of miner running, pc freezes and that's it.
Tried to swap slots, and the problem seems to be in second PCIe x16 slot.
Tried different BIOS versions: 3.20 (stock), 3.30, 4.40, 2.50. Same shit.
On 4.40 the AMD PBS section appeared again (mentioned in the thread before), I tried to switch to 2x8 mode, and that gave me 5 short beeps and black screen (no signs of boot) if there are cards connected with riser.

Notes:
1. Cards are with hacked bios. All are working fine, tried them on another working rig.
2. One card is connected directly into the first PCIe x16, without riser. It's just more convenient. Pity if that's the cause of the problem.

Any ideas how to make 6th card work stable?