If you're migrating to office 365, you may notice this error message, irrespective of your modality of migration, i.e.e cutover, IMAP or Hybrid mode migrations. The exact error message read:
The Subscription for the migration user USER@DOMAIN.COM couldn't be loaded. The following error was encountered: A subscription wasn't found for this user.
There are two issues at play here, one is that there was a large number of licensing request last week that “overwhelmed” the Office 365 licensing service for a while according to , while this week, there is a code rollout to the exchange Online service which is affecting migrations to “limited number of customers.” Both of these items should be visible in the Service Health portion of your Office 365 tenant Dashboard.
IF you are a affected customer, then you don't have too much of an option at the moment, besides to hurry up and wait, while the issue is remediated, and keep on trying.