Disabling OWA Form Authentication caused ECP to be inaccessible

Hi,

I'm trying to publish Exchange 2013 OWA via ISA Server 2006 using its form authentication. For that, I disabled the OWA Authentication Form to avoid being asked to log in twice. However after disabling it, I could not longer access ECP. When opening the ECP, I would be shown the normal OWA login form but upon logging in, I would be shown an error page with the link in the URL as "https:/[servername]/owa/auth.owa"

If I re-enable OWA Authentication using "Set-OwaVirtualDirectory" command, I am able to access ECP again.

Any idea what is happening?

Thanks.