Sometimes an lxc container will refuse to start, usually after something goes wrong on the host. The key is to manually start the container and wait for some timeout or something, then the container will start properly.
Update: this does more or less the same but with less work.
pct list # note the vmid, replace 100 101 below as appropriate
for m in 100 101; do lxc-start -n $m & done
# after a minute or so you'll see the background tasks finish in the console
pct list # lxc containers should be running
# Note, that `systemd` will still show the tasks as failed since it didn't
# start them, so now lets stop/start them properly
for m in 100 101; do pct shutdown $m; pct start $m & done
The following is still valid but I now use the above method.
[root@proxmox1 ~]
# pct start 125
pJob for pve-container@125.service failed because a timeout was exceeded.
See "systemctl status pve-container@125.service" and "journalctl -xe" for details.
command 'systemctl start pve-container@125' failed: exit code 1
[root@proxmox1 ~]
# /usr/bin/lxc-start -n 125 -F
# good long wait, over a minute
# login and then `poweroff`
[root@proxmox1 ~]
# pct start 125 && echo $?
0
tl;dr
lxc-start -n 125 -F- login and poweroff
pct start 125