If other users have an event open you will see this message "Unable to edit event as another user already has it open.." if you try to edit the same event. It is possible that after a power failure or not shutting down Windows or Schedule it correctly that any open events could be in a locked state and only available for read only. If you are sure the user has closed the event you can reset this lock.
You can see the user and computer that has the event open under the Activity tab in the event and filter for the action Event_Open.
Get the same user that had the event open previously to reopen the event on the original computer. Then close the event to remove the user lock.
As an Administrator, from the Tools menu, select Activity Log,
In the 'Filter by Action' list, choose 'Event_Open',
In the 'Filter by ID' list, choose the ID of the event if known,
Press the Search button to see the lock entry then select from Options... Delete All to remove the user lock entry.
Last updated, 27 January 2013, 15:40