the shared calendar's connection status shows "Disconnected" in the Outlook Connection Status

Hello @ANdrea fr,

Welcome to Microsoft Q&A!

Thank you for providing such a detailed and technical description of the issue you are facing with your Outlook for Microsoft 365 client. I understand how frustrating it can be when a core function like a shared calendar connection fails silently, especially when your primary mailbox is working correctly.

Based on the information you've shared from the Outlook Connection Status and ETL traces, it appears you are dealing with a credential or token caching issue specific to secondary connections. This is a known, though not common, scenario where a stale or invalid OAuth token for a shared resource fails to be renewed, causing a silent disconnection.

Here are the steps we recommend to resolve this, starting with the most likely solution:

Step 1: Clear Outlook's Cached Credentials

The most effective way to resolve a token mismatch is to force Outlook to re-acquire new authentication tokens for all your connected services. This can be done by clearing the stored credentials in Windows.

1. Completely close Outlook and all other Office applications.
2. In the Windows search bar, type Credential Manager and open the application.
3. Select Windows Credentials.
4. Under the "Generic Credentials" section, look for any entries that mention MicrosoftOffice or MSO.
5. Click on each of these entries and select Remove. Confirm the deletion.

Once you have removed all relevant credentials, restart your PC.

Step 2: Force Re-Authentication

After your PC has restarted, open Outlook. The application should now prompt you to enter your password. This is a critical step, as it forces Outlook to perform a clean authentication and acquire new, valid tokens for all your connections, including the shared calendar. 

Why This Works

Outlook maintains separate authentication sessions for different resources (e.g., your primary mailbox, a shared calendar in another tenant). The token for the shared calendar may have a different expiration policy or be subject to different security rules (like Conditional Access Policies on the other tenant). By clearing the cache, you eliminate the stale token that was causing the connection to fail, allowing Outlook to start a new, successful authentication flow.

We hope this resolves your issue. Please let us know if the problem persists after following these steps, and we can explore more advanced troubleshooting options. Warm regards   

Chloe L. | Microsoft Q&A Support Specialist

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