KB2969: Veeam Backup for Office 365 Complete Permissions
https://www.veeam.com/kb296926.06.2019 · Sharepoint Permissions Configuring Permissions for SharePoint Online. Grant Veeam service account with the SharePoint Administrator Role in Azure Admin Center. In the SharePoint Admin Center open [Policies] > [Access control] and set "Apps that don't use modern authentication" to "Allow Access". 2. Configure the App password