LTSSM in State 0xF, but BIOS does not POST

I have an Arria 10 SoC connected to a PC's PCIe Gen3x16 Slot. The connector is an FMC to PCIe x8 connector. I can see using Signal Tap that the FPGA's LTSSM in state 0xF. However BIOS does not POST.

With the FPGA disconnected, the PC boots up just fine. Also if I replace the FPGA with another card, the PC boots up just fine.

The exact same setup, same design, cables, has worked successfully on another motherboard.