Just had a reply from ASRock Support:
Hello,
1. Please try to borrow more RAM to test with.
2. Try increasing virtual memory in Windows to at least 16GB.
3. The Molex power connectors on the motherboard must be used, even with powered risers. I know you tried this already.
4. Please try to test with Windows 10 version 1703.
Any change?
Kind regards,
ASRock Support
Trying this now
I don't care what asrock support says, the molex power connectors aren't to be used UNLESS you are using Unpowered risers, not powered risers.
I have both the older version of the h81 and the newer r2.0 neither of which do I power the molex on the motherboards and they work as intended