You probably need to properly configure the domain for OWA. There is a right way and a wrong way to do this. If you configured this through IIS, you did it the wrong way. Reverse whatever you did and do this:
From Exchange Management Console, open up Server Configuration -> Client Access. Under the OWA tab, right click the OWA application and go to properties. Click on the authentication tab. Use forms based authentication, user name only, and browse for the proper logon domain.
If this still doesn't work, you might have a DNS issue or are lacking the proper prereqs at the site (like the global catalog server). How many sites do you have? What does your Active Directory setup look like? Can you post IIS logs so we can see what the error is there?