Gaël, I cannot reproduce the issue with either the deb or snap LXD.
The failure is occurring in code that runs inside the container: "juju bootstrap" connects to LXD and starts a container, and then injects Juju code into the container; that code is what is failing to connect to LXD.
If you could run bootstrap with "--keep-broken", that initial container will be left behind. This would allow you to enter the container and find out why it cannot connect to the LXD socket.
Gaël, I cannot reproduce the issue with either the deb or snap LXD.
The failure is occurring in code that runs inside the container: "juju bootstrap" connects to LXD and starts a container, and then injects Juju code into the container; that code is what is failing to connect to LXD.
If you could run bootstrap with "--keep-broken", that initial container will be left behind. This would allow you to enter the container and find out why it cannot connect to the LXD socket.