The "This is an invalid operation. Cannot add owner as delegate." message is just a warning message and can be ignored, as this message is just advising that the user “Cannot add owner as delegate”.
The reason being that the user has assigned themselves as a delegate. A user doesn't need to assign themselves as a delegate, as the user is the Owner of the mailfile and therefore doesn't need delegated rights.
This can be checked under the Notes User Mail | More | Preferences | Access & Delegation Tab | Access to Your Mail & Calendar sub tab.
