Outlook Calendar Error Message: One or more items in the folder won't synchronized

Good day Martin, 

Just to gently clarify before going any further, this forum is separate from Microsoft Support. I don’t have the ability to access or investigate individual accounts because this is a user‑to‑user support forum.  

This error occurs when a recurring calendar event contains conflicting occurrences. For example, two occurrences may have been moved to the same day, their times may overlap, or an occurrence may have been changed in a way that disrupts the sequence of the recurring series. An outdated Calendar cache in Outlook for Mac can also cause Outlook to keep syncing an inconsistent version of the event. 

Please try the following steps: 

Option 1: Check the recurring event in Outlook on the web 

Sign in to Outlook on the web and open Calendar. 

Find the recurring event that was recently created or modified. 

Open the entire series and review any occurrences that were moved to a different date or time. 

Check for duplicate dates or overlapping occurrences. 

Correct the conflict and save the series. 

If you cannot identify the problematic occurrence, delete the recurring series and create it again.  

Option 2: Clear the Calendar cache in Legacy Outlook for Mac 

Note: Before clearing the cache, please confirm that your recent calendar changes are visible in Outlook on the web. 

Open Calendar in Legacy Outlook for Mac. 

Right click the affected Exchange calendar and select Properties. 

Open the General tab and select Empty Cache. 

Allow Outlook to re-download the calendar data from the server. 

Restart Outlook and check whether the error returns. 

Option 3: Reset the Outlook account 

If the issue continues: 

Confirm that your emails, contacts and calendar items appear correctly in Outlook on the web. 

In Outlook, go to Tools > Accounts. 

Select the affected account. 

Choose Manage > Reset Account. 

Allow Outlook to download the mailbox data again and then restart Outlook. 

In addition, Legacy Outlook for Mac relies on Exchange Web Services for Exchange Online connectivity. Since Exchange Web Services was retired on October 1, 2026, Microsoft recommends using the new Outlook for Mac for improved reliability, performance and security. You may consider switching when convenient. For more information, please refer to: End of support for legacy Outlook for Mac | Microsoft Support 

If you have any questions, please don’t hesitate to reach out at any time. 

Thank you for your patience and understanding. 

If you have any extra questions about this answer, please click "Comment".