Ok, this is so straightforward.
What happened is that with older versions of LXD, you could specify localhost but now you can’t.
But what happens if you have some lingering old proxy devices that still mention localhost?
In that case, you are stuck because you cannot perform any other configuration on this container!
LXD will not accept any change to the configuration of the container because after any change, the configuration will still be invalid due to the localhost.
But why can’t you just remove the offending proxy device (with localhost)?
Because it looks like you have two offending proxy devices, and with lxc config device remove ... you can remove only one device at a time. But you cannot remove any of the two, because the other will still be in there.
What you need, is a workaround to sort this issue.
Can you run
debian@br-lxd:~$ lxc config edit haproxy
and edit out both the localhost references in your proxy devices?
Does that command work for you or does LXD complain even with lxc config edit haproxy?