The O365 Tenant is a Multi-Geo Region tenant, with, for example, the main tenant in the EU region and a satellite tenant in the NA regions. The MNE account for migration, together with Admin Pool accounts is created in the EU region.
You see below a set of error messages with 5 attempts:
DEBUG: PS > Add-MailBoxPermission -Identity user.name@domain.com -User Notes Migrator Admins Role Group -AccessRights FullAccess -AutoMapping False -Confirm False
DEBUG: Sleeping 30 seconds after attempt 1 to execute PowerShell command.
ERROR: [5081-174-12-00000000]Unable to grant the required Full Access permissions for the target mailboxes. Aborting the task.
CMigAppTaskApp::SetAdminPermissionOnMailbox() - PowerShell Result: Error: Error on proxy command 'Add-MailboxPermission -Identity:'user.name@domain.com' -AutoMapping:$False -Confirm:$False -User:'Notes Migrator Admins Role Group' -AccessRights:'FullAccess'' to server DM5PR2201MB1705.namprd22.prod.outlook.com: Server version 15.20.3391.0000, Proxy method PSWS:
Request return error with following error message:
The remote server returned an error: (401) Unauthorized.. [Server=FR2P281MB0089,RequestId=88cb8a08-4ff3-424e-a3da-48f7c68943a9,TimeStamp=01/01/2020 10:10:10 PM]