If your Exchange 2010 OWA is loading with a blank page internal and external it may be due to a recent Exchange update. After installing this update, when you browse to your OWA page the URL will change to something similar to https://mail.domainname.com/owa/auth/logon.aspx?url=https://mail.domainname.com/owa/&reason=0
To fix this, open Exchange Management Shell, and enter the below commands
- CD "C:\Program Files\Microsoft\Exchange Server\V14\Bin"
Once the updatecas.ps1 command has finished just reload your OWA page.
Reference : http://www.msexchangeblog.nl/2010/05/11/blank-page-in-owa-fix-for-exchange-2010/