It could be that a download operation was initiated by the daemon and is stuck due to network problems. If that's the case "systemctl restart lxd" should unstick things.
If the problem persists after restarting the daemon, then my guess would be some MTU problem which is the usual suspect for such random but repetitive problems.
You can also try "lxc image export ubuntu:12.04 ." which will perform the same image download directly in the client rather than through the LXD daemon.
It could be that a download operation was initiated by the daemon and is stuck due to network problems. If that's the case "systemctl restart lxd" should unstick things.
If the problem persists after restarting the daemon, then my guess would be some MTU problem which is the usual suspect for such random but repetitive problems.
You can also try "lxc image export ubuntu:12.04 ." which will perform the same image download directly in the client rather than through the LXD daemon.