“The delegates page is not available. Cannot access Outlook folder” error message when a user tries to configure a delegate by using Outlook 2010 / 2013 in an Exchange Server 2010 / 2013.
After some research I found out that a corrupt Outlook rule was blocking the delegate list. A corrupt server rule was blocking the delegate list enumeration. After deleting the corrupt Outlook rule the user could set delegates again.
Outlook Delegates Page Not Available