Problem with forms authentication in OWA

Hi

I have a problem with exchange 2013 standard

When I enable forms authentication on OWA, after logging in I get a 404 file not found error.

Forms authentication works fine on ECP.

I noticed that the login url has a ReturnUrl on the querystring which is double url encoded.i.e. I am getting

https://centaur.patriot.local/owa/auth/logon.aspx?replaceCurrent=1&url=https%3a%2f%2fcentaur.patriot.local%2fowa%2flogin.aspx%3fReturnUrl%3d%252fowa%252f%253fbO%253d1%26bO%3d1

If I unescape the ReturnUrl and put that in the browser,(like the following), it works:

https://centaur.patriot.local/owa/auth/logon.aspx?replaceCurrent=1&url=https%3a%2f%2fcentaur.patriot.local%2fowa%2flogin.aspx%3fReturnUrl=/owa/?bO=1&bO=1

So either IIS is redirecting twice or the redirect url is incorrect.

Any ideas on how to fix this?