Stuck in a boot loop with RTX PRO 5000 Blackwell on x299 Motherboard

Hi all - can't seem to find any relevant information related to my build. I'm trying to repurpose an old x299 motherboard to use for local AI and other graphics workloads. I just got the RTX PRO 5000 Blackwell, but I can't seem to boot on this platform (running on an X299 Designare EX with a 10980XE CPU).

I've tested different power cables, BIOS versions, PCIe Slots, etc, and I get stuck in a boot loop before the machine even posts. it just restarts over and over in quick succession, with code 03 (or D3?) on the MoBo.

Any suggestions? I've tested the card on a Z490 Motherboard, which is also PCIe 3.0, so that's not the issue.

Is my MoBo simply too old for the newer Blackwell cards? It works just fine with RTX 40xx series cards, btw.

Thank you!