Seeing this error indicates that your migration failed checking source credentials. The Client Access Server version does not match the accessed resource's Mailbox Server version. Use Autodiscover with the address of the resource being accessed to determine the correct URL to use to access the specified resource.
This error will occur if the incorrect OWA URL is set within the project or the Admin account that is being used to perform the migration is on a different version of Exchange than the one that the user mailbox is on.
For example, the Admin is on Exchange 2007 and the user is on Exchange 2013.
If this error is being reported for all users in the migration, create a new Admin account on the endpoint that is reporting the error using the steps in this article: Set Up Exchange Administrator Account.
If this error is only being reported for some of the users, then it is likely that these specific users are on a different version of Exchange. To resolve this, clone your project and move the users with the error to the new project. Then create a new admin for the separate Exchange environment.
For more information, read the TechNet article here.