I was working this same exact issue. I have found a solution, but the install instructions were incomplete (see below).
BTW, I went thru all the MS described solutions for the OWA loading issue. http://support.microsoft.com/?kbid=280823 nothing worked. The reset HighWaterMarks procedures ended up elimimating the OWA form login screens.
I installed the Microsoft Patch http://support.microsoft.com/?id=831464. The description of this download was noted as a fix for the OWA loading issue in the SBS2003 downloads, but in the patch description symptoms were less descriptive -- but go ahead and install it.
However, after you install this patch - you must ALSO follow the workaround instructions in the same patch to **clear the server cache**.
Install patch 831464. then:
To delete the cached files on the server, follow these steps:
1. Open the IIS Temporary Compressed Files folder.
The default path is %windows%\IIS Temporary Compressed Files.
2. Delete all the files in this folder.
3. At a command prompt, enter IISReset to restart all IIS services.
Now OWA works! I went out and joined this forum as a result.
