For the record, I’ve seen this exact problem as well with recent lxd using the Snap package installed by following the instructions at Linux Containers - LXD - Getting started. I’ve been generally annoyed with having to use snapd for this so I’ve recently switched over to the (not yet Debian-included) upcoming lxd package for Debian from https://apt.calenhad.com/
Interestingly enough, I’ve had the exact same problem with lxd installed from that package though. After email conversation with Mathias Gibbens (thanks!), the root cause seems to be the iptables package on my system, causing lxd to be confused. And iptables is in turn installed because it’s a dependency by the docker.io package, i.e. Docker… 
So the problem will probably go away if you uninstall docker.io, but until then the workaround described above can perhaps be of help for other people who also use Docker + LXD on the same machine.