Outlook shows conflict errors when updating or cancelling meetings and/or keeps showing old meeting reminders

To mitigate the calendar conflict issue, you can use the Clear Offline Items for your calendar. These steps clear the calendar items in the Outlook store (OST) on your local machine. After performing these steps, Outlook will sync clean copies of the meeting items from the service.

To clear offline Calendar items, follow these steps: 

Close all Office Applications.

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

Select Properties.

On the General tab, select Clear Offline Items.

Select OK.