It could be cause by this libvirt error: libvirtd[18329]: Failed to open file ‘/sys/class/net/vethSUT3FA/operstate’: No such file or directory

Fix:
brctl show
bridge name bridge id STP enabled interfaces
virbr0 8000.525400a97813 yes virbr0-nic

vim /var/lib/lxc/my_server/config
lxc.network.link = virbr0

lxc-start -n my_server -d