I assume your cluster was of 2 nodes, is that correct?
With the LXD 4.0.x series we now prevent the internal dqlite database to be clustered when you have only 2 nodes, it will be clustered starting at 3. That’s because with 2 nodes, if you lose one then you can’t get the other working, since quorum is lost. With LXD 4.0 we also have recovery tooling to fix situations like the one you’re reporting.
However the LXD 3.0.x series is quite old in that regard and its clustering implementation is much more limited than it’s today.
If it’s a viable option for you, I’d suggest to stop LXD, move away your database directory (e.g. mv /var/lib/lxd/database/ /some/back-up/dir/), then restart LXD (which will be empty) and import your existing containers with lxd import.