Having the same issue here. SSH won't start on a brand new Ubuntu 20.04 LXC container. I also tried a Ubuntu 18.04 container, as well as a 20.04 privileged container, all having the same issue.
Host info:
pve-manager/7.0-10/d2f465d3 (running kernel: 5.11.22-3-pve)
I can attach to the LXC...