The other day, a user was trying to give his new personal assistance Delegate permissions to his mailbox. However, whenever he clicked on Delegate Access:
He would receive an error message stating: “The Delegates page is not available. Cannot access Outlook folder.”
After some troubleshooting and digging around, it turned out that this was being caused by a particular Outlook (Inbox) rule! I am still to investigate why this rule was causing this problem and why would any rule prevent configuring delegates, so this is still a mystery...
If you are experiencing the same problem, simply disable any existing rules one by one until you are able to configure delegates. Alternatively, you can start Outlook by typing “Outlook /CleanRules” in the Start Menu to start Outlook and delete all rules (probably not recommended).