Recently upgraded to CU3. I'm not sure if my EAC problems are due to that or something else. I could have swore this worked before, but I am not 100% positive.
If I go to "Recipients > Mailboxes > Properties on a mailbox > Mailbox Delegation" it says "Please wait...". It stays like that for about 15-20 seconds, then finally comes back with: "error. Your request couldn't be completed. Please try again in a few minutes". I hit OK and the boxes for Send As, Send on Behalf, and Full Access are all empty.
I can view permissions on that same mailbox with PowerShell.
Also, if I do the same process for a Distribution Group, then that works okay in EAC!