Just wanted to echo that this has stopped me for starting my LXC containers after updating Proxmox today.
This fixed the problem for me for now. Thank you very much for posting this!
Maybe this can be fixed by removing the code that's causing this issue in /usr/share/perl5/PVE/LXC.pm:
PVE::Tools::fchownat(fileno($mount_fd), '', $uid, $gid, PVE::Tools::AT_EMPTY_PATH)
or die "failed to propagate uid and gid to mountpoint...