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