Ubuntu 20.04 container failing to startup in Proxmox
Solution 1:
I think your issue is the same as the next : https://github.com/lxc/lxc/issues/2754
There ssems to be a two solution.
solution 1. Use PIDFile=
[Service]
Type=forking
PIDFile=/run/lxc-%i.pid
ExecStart=/usr/bin/lxc-start -d -p /run/lxc-%i.pid -n %i
...
solution 2. Change the type from "forking" to "oneshot"
-Type=forking
+Type=oneshot
+RemainAfterExit=yes