When copying a container to a different server that has snapshots which includes device information such as proxy, this information is lost from the snapshot during the copy.
So lets you have a proxy setup like this in devices,
[proxy-8080] => Array ( [connect] => tcp:127.0.0.1:80 [listen] => tcp:0.0.0.0:80 [type] => proxy )
Then create a snapshot and then run
lxc copy c1 server2:c1
On the remote server, the snapshot no longer has the proxy device, so it was lost during copy.