1. If these workers have been successful in the past, they may need maintenance.
2. If these are new workers and never had a successful migration, then it's possible there's an issue with Outlook.
1. Reboot the worker machine. If this does not resolve the problem, then uninstall and reinstall the migration worker software on the failing machines.
2. Uninstall Outlook, then apply the latest patches and fixes from Microsoft from this URL: https://docs.microsoft.com/en-us/officeupdates/office-updates-msi
