The issue is indeed resolved by using a LXD storage-pool type that is compatible with foundationDB (cf. https://lxd.readthedocs.io/en/latest/storage/).
For example, LVM with underlying ext4 works.
(For those, who - like me - want to preserve their precious container content but just get foundationDB to work:
lxc storage create lvm lvm
lxc profile copy default fdb
lxc profile edit fdb # change ‘pool’ value to ‘lvm’ in editor
lxc stop my-container
lxc copy my-container fdb-container --container-only --profile fdb
lxc start fdb-container
You may have to apply the advice from Trying to make docker image; fdbcli says database not created after that.
)