Authentication error from Desktop app when trying access Copilot add-ins

hi,

We have a user assigned with the following licenses:

• Exchange Online
• Microsoft 365 Apps for Business
• Microsoft 365 Copilot

When the user clicks the Copilot button from any of the Desktop Apps (e.g., Outlook, Word), the following error appears:

Correlation Id: ff9403b6-0a4f-e378-3171-97233ff73c2e
Timestamp: 2025-11-09T08:07:57.000Z
DPTI: 49c651f43c01bb0bc2a042d8934595ab1937aa244c88ee4
Message: Access denied for the resource.
Tag: 4usqa
Code: 3399614475

What we have already checked:

• ✅ Copilot add-in is enabled
• ✅ Outlook version is the latest
• ✅ Repaired Microsoft 365 Apps for Business
• ✅ Deleted and re-created the Outlook profile
• ✅ Modern Authentication is enabled
• ✅ Windows is fully updated

Important note: Copilot works fine in Outlook Web, but not in Outlook Desktop.

Regards, Vladimir