In Exchange 2013 your users might encounter the following error when trying to reply or create a new email or choose a recipient from the address book:
“Error: Your request cant be completed right now. Please try again later.”
On your Exchange 2013 Mailbox Server, if you open IIS and navigate to Exchange Back End -> OWA ->Authentication Methods and enable Anonymous Authentication.
Once complete perform an IISRESET on the server:
Now going back to OWA you can now reply to emails and choose users from the address book as shown below:
You can reproduce this by simply disabling the above authentication and refreshing OWA.
Hope it helps.