This handy KB will show you how to re-create Exchange Server 2013 OWA IIS directory and reset all settings.
This is a very handy option that can fix many Issues related to OWA.
This fix can be used to fix all IIS directories on Exchange like ECP, OAB, EWS and Autodiscover.
To start open EAC and go to server -> virtual directories
Select the server and the directory you need to reset and click on the reset button
Close when reset completed
Restart IIS using IISreset