Thought I just share a bug I found in Exchange 2013 CU5, that has been confirmed Microsoft Exchange Technical Support. My customer have a mix Exchange 2007 and Exchange 2013 organization. When CU5 where installed, OWA redirection when FBA is turned off on both 2007 and 2013, is broken.
Short-term solution before there is a fix: Enable FBA on both 2007 and 2013.