The server has rejected the client credentials.
When configuring the "Enable restore operator authentication with Microsoft credentials" feature, a certificate was selected from the Certificate Store, or a certificate was imported. However, that certificate was not added to the Trusted Root Certification Authorities.
The application certificate within the Restore Portal settings has been updated, but it has not been added to the application settings of Azure Active Directory.
Make sure that the certificate is added to the Trusted Root Certification Authorities.
Reference: Authentication Settings
If the certificate is updated within Veeam Backup for Microsoft 365, it must also be added in the application settings through the Azure AD admin center.
Reference: Restore Portal Settings
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