I just wanted to report facing the same issue, and the exact same error message for the same BFD device number as OP.
In my case, I'm attempting to pass one of the USB 2.0 Controllers from my Supermicro X10SDV-TLN4F motherboard (featuing a xeon-d 1541 CPU). Since it is the same BFD number as OP, I wouldn't be suprised it's similar hardware.
00:1d.0 USB controller: Intel Corporation 8 Series/C220 Series Chipset Family USB EHCI #1 (rev 05)
After the device is hidden in grub.cfg it is correctly reported by xl pci-assignable-list. The error I get on VM start-up after assignation is:
Internal error: xenopsd internal error: Cannot_add(0000:00:1d.0, Xenctrlext.Unix_error(30, "1: Operation not permitted"))
FYI, passing the entire USB Controller was my 2nd attempt at passing a USB Bluetooth dongle which I was unssuccessfull in acheiving due to similar issues as in https://xcp-ng.org/forum/topic/266/usb-passthrough-test-reports-in-7-5rc1.
The RMRR explanation makes a lot of sense to me as well. I wouldn't mind passing more BFD devices to the same VMs if it fixes the issue, but I haven't found a combination that works.
I was hoping someone extrapolated a way to make this works in XCP-ng from the work done in the Qubes community as mentionned by @hkparker.