This error from ownCloud indicates that you’ve jumped too far ahead in one step as you restore a backup. If you’re creating a new Mail-in-a-Box (not restoring from a backup), this should not arise since we’ve recently made it so that we will do several step by step upgrades as necessary. But we probably don’t have this working for backup restores yet.
You could do an intermediate upgrade e.g.:
$ export TAG=v0.19
$ curl -s https://mailinabox.email/setup.sh | sudo bash
This would install version v0.19 of Mail-in-a-Box.
I’m not sure what intermediate version(s) you would need to go through for whatever version you are at now in the backup, unfortunately.