Thanks, that makes things much clearer. The problem is confusion between
The former exists on your system and is configured through
lxc-net, the latter is disabled in the config file you pasted above.
LXD is looking for
lxdbr0, not for
lxcbr0, causing the error.
To make LXD use your existing
lxcbr0 bridge instead of its expected
lxdbr0 one, you should just run
lxc profile edit default and then replace
lxcbr0 in the configuration, saving the result should then make things work.
An alternative would be to discontinue the use of your old
lxcbr0 bridge on this system, and re-enable
dpkg-reconfigure -p medium lxd, in which case you wouldn’t need to change the default profile.