That suggests the apparmor profile of that source instance is preventing a mount namespace operation required by lxd-p2c.
You could likely temporarily change the profile to unconfined during that transfer with lxc.apparmor.profile=unconfined (actually, 3.0.x may be lxc.aa_profile instead).
Thank you. That would probably solve my problem, but currently I don’t know how to unconfine the container without restarting it (or maybe change its aa mode to complain). Looks like it’s time for me to finally get some knowledge about apparmor .