You may have corrupt or missing OWA files.
I'd try uninstalling / reinstalling OWA. Here's how from TechNet:
To correct this behavior, remove and reinstall the OWA components from the
Exchange Server computer by performing the following steps:
1. Back up the \Exchsrvr\Webdata directory on the Exchange Server computer.
2. Run the Microsoft Exchange Server Setup program, and choose the Add/Remove
option.
3. Click to clear the "Outlook Web Access" components check box, and continue
with Setup to remove the OWA components.
4. Delete the \Webdata directory.
5. Run Setup again, and click to select the OWA components.
6. Continue with Setup to perform a clean installation of the OWA components.