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.