Recently, some customers could not add additional delegates after applying Outlook Update 913807. If they try to add a delegate in Outlook, the following error message appears: "The delegate settings were not saved correctly. Unable to activate send-on-behalf-of list. You do not have sufficient permission to perform this operation on this object". Removing update 913807 solves this problem. However it is not recommended as new update package will include this update.
The better solution for this issue is to add the "Write Personal Information" permission to the SELF account for the user trying to add the delegate.
1. Start Active Directory Users and Computers (ADUC).
2. Click View -> Advanced Features
3. Locate the user's OU. In the right pane, Right Click the User who wants to add Delegate in Outlook and Click Properties.
4. On Security tab, Locate the SELF account and Set "Write Personal Information" to Allow
5. Click OK
Now, the user can add delegate properly. By the way, if you want to let userB help userA add delegate, besides "Full Mailbox Access", please don't forget to grant this "Write Personal Information" permission to userB.