As discussed in JBEAP-3515, it's sometimes possible for the :migrate operation to end up with a migration error. This is different from migration warning – in case of a migration warning, the migration is basically successful, almost everything was migrated and the user just has to deal with the warnings. In case of migration error, something ended up horribly wrong, the entire migration is rolled back and nothing is migrated. This situation is marked by an error message Migration failed, see results for more details., and there are more details in the server response.

As far as I know, we never hit that situation in our testing (rhatlapa@redhat.com, mnovak1@redhat.com, ochaloup@redhat.com?), so we can't really provide an example or more details. Maybe emartins@redhat.com knows more?