If OWA is in fact enabled, then try preceding the username with the domain name and a forward slash in the logon prompt. For example, instead of typing just JDoe, type MyDomain\JDoe -- obviously replacing the domain name and account name with the actual values.
If that still does not work, try resetting the password from the server. While this may seem somewhat odd, I have actually seen instances (on more than one occasion) where the user changed his/her password from a workstation, successfully logged onto the domain using the new password from the workstation, but couldn't access OWA. Resetting the password at the server actually fixed it. Strange but true.
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