Ensure that any administrative mailbox is mailbox enabled: The migration is configured using an administrative account to access the mailbox. The administrative account does not have a mailbox associated with it.
Mailbox-enable the administrative account used. The administrative account is only used to log in to other mailboxes. However, due to a known Exchange limitation, the administrative account must be mailbox-enabled for MigrationWiz to be able to create item in user mailboxes.
After mailbox-enabling the administrative account, initialize OWA. You can initialize OWA by logging in to OWA. If OWA has already been initialized, you will see the Inbox. If OWA has not been initialized, you will be prompted to select your region and time zone.