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BugZero found this defect 1558 days ago.
When adding a Microsoft 365 organization with Modern Authentication to Veeam Backup for Microsoft 365, there is an option to enable "Allow for using legacy authentication protocols." When this checkbox is not selected, Veeam Backup for Microsoft 365 will use a mode referred to as Modern App-Only Authentication.
For a complete list of considerations and known limitations of Veeam Backup for Microsoft 365, review: Considerations and Limitations