While the user Calendar delegation is configured, the delegate cannot add more participants or change the appointment's data. KOFF generates You don't have appropriate permissions to perform this operation 0x80070005 error.
The delegated user has only access to the Calendar folder.
The delegate user has been given delegate access to the primary user's calendar, but not to their email. This can prevent the delegate from sending emails on behalf of the primary user. Changing Calendar's appointment data such as adding new attendees or modifying event timeframe, requires sending an email with updated data. Because the delegated user doesn't have access to INBOX, the permissions error from Kerio Outlook Connector is being generated.
Grant INBOX access for the delegated user.
For more information, please refer to Using Delegation in Kerio Connect Client.
The error is no longer shown in KOFF and the Calendar events can be modified on behalf of the user.