When trying to update or cancel a calendar meeting item I get the message "One or more items in the folder you synchronised do not match"

This issue happens when there is a local conflict of the calendar item in Outlook Desktop. The server copy of the calendar item does not have a conflict.

Workaround 1:

To mitigate the issue, you can use the Clear Offline Items for your calendar. This step clears the calendar items in the Outlook store (OST) on your local machine.   

To clear offline Calendar items, follow these steps: 

1. Open the Calendar pane in Outlook, and right-click the affected Calendar folder.

2. Select Properties.

3. On the General tab, select Clear Offline Items.

4. Select OK.

After you clear offline items, Outlook will remove the existing items in that calendar then re-sync and repopulate that calendar folder with the calendar items from the server.

Workaround 2:  

Make the meeting updates in Outlook for the Web which does not have a conflict.

Related Link

Microsoft Support: Outlook shows conflict errors when updating or cancelling meetings >