"No Entry" sign on boot.

I have messed something up in my hackintosh. It's running Catalina, and has been rock solid stable now for ages. Really happy with it.

For various reasons, which I won't bore you with, I decided to make it dual-boot. I have installed windows on an NVME stick hosted on a PCI adapter. I was careful to remove all Mac SSD drives and disconnect APFS data volumes prior to doing this.

I have now reinstalled the Catalina M.2 SSD and the two data volumes (one of which hosts the user data).

My Opencore EFI boots fine, loads up most of the boot, then drops to an unceremonious "No Entry" sign on the screen.

I don't think I've done anything particularly dumb, but obviously I've messed something up.

If someone could nudge me in the right direction it would be very much appreciated.