Comment 1 for bug 1860743

This *might* be due to host key caching that the nova-cloud-controller is now doing. Please could you try to clear the host key cache on the nova-cloud-controller(s) unit using the juju action:

    juju run-action nova-cloud-controller/0 clear-unit-knownhost-cache

This needs to be run on all the nova-cloud-controllers.

This will re-find all the the SSH keys on all the nova-compute units and re-share them. If this fixes the problem, then the issue is around not clearing the cache and re-seeding the keys when the migration network is changed.