I'm on W10PRO, latest drivers, etc... everything uptodate.
Of course I tried to lower OC and so on, without success.
What's in common with those 2 RIGS: having RX580 NITRO+ with HYNIX memory (1st rig with 12 of those cards, the others with 6 of them + 6 GTX).
Well I am on Win10 1709 and my Rig has 13 RX580. Running at total 398 mh/s on ETH/nanopool flawlessly.
Please enlighten me on how it is you were able to break the 8 AMD GPU barrier in Windows 10?

There is no such thing as a barrier in WIN10. That used to be implemented in the drivers. You need upto date win10 (1709), latest AMD drivers (18.2.3 Feb. 2018) - install ONLY driver!, compute mode switcher (
https://bitcointalk.org/index.php?topic=2815803.0), a max number of 13 similar cards and a mainboard with enough PCIE slots. Then you're good to go. My rig with 13 cards works flawlessly and the output is close to 400 mh/s on ETH/nanopool. no probs at all.
The 13 card barrier is because of the BIOS of my mainboard (Asus B250 expert mining). Power draw on the wall is 1750 watt for the rig including monitor (my rig is like an open cube built with aluminum profiles, with wheels and inside with 3 rows of cards on top of each other).