Expected XML node type was XmlDeclaration, but actual type is Element: This normally indicates that the server is unavailable or under stress, and as a result is returning unexpected XML content when accessed using the EWS (Exchange Web Services) API.
Resolution:
Check if the Exchange server is responding and can be logged into using OWA. Resubmit the migration if it failed, or submit your mailbox in Retry Mailbox Migration Errors mode after the mailbox has successfully migrated.