I ran into the same issue and was able to get the console working by changing console mode to /dev/console in Proxmox options:
I can get the login prompt through the Proxmox UI, but I can’t log in. I’ve tried root / (empty password), root / nixos, nixos / nixos and a bunch of other variants. I’ve tried setting the password from Proxmox container setup, but NixOS seems to ignore that.
This is using the pre-build proxmox container from hydra (specifically 24.05 / 264827929).