Running in to the same issue as described here:
https://forum.proxmox.com/threads/bootable-nvme-install-on-old-hardware-made-easy-with-pcie-adapter-and-clover.78120/post-713817
releases newer then V5121 just boot in to a black screen. I'm using a Dell R730 with an ASUS Hyper M.2 x16 Gen 4 card. Where clover is used as a linux bootloader, as the dell bios does not support NVMe booting.
When I boot V5121 everything works fine and as expected.
Clover is installed on the ISDM (dell built-in sd card slot).