There are various causes for this error. It would be helpful to know if you are running this on WIndows Server 2003 Standard/Enterprise, or if you are using Small Business Server (SBS) 2003, and what service packs do you have installed for both Exchange and Windows.
Also, are you able to access email using Outlook rather than OWA? If not, then you may likely have an issue with your Information Store and should check whether it is even mounted.
Microsoft has published a Knowledge Base article that relates to this error message. The article describes problems with the application pool identity. Please review the article at http://support.microsoft.com/?id=829167. If it relates to your situation, then consider making the suggested changes and let us know the results.
I am agree with Brad, but i also have faced this problem. I have installed a fresh Windows Server 2003 and Exchange Server 2003, i have not created ADS in this server but called it from our main server. At that time i faced this problem.
When i searched google for this reason, somewhere i found that this may probably possible if SMTP service/file is unavailable or currupted. You just try to uninstall SMTP and then again install it. For that follow steps:
Go to Control Panel > Add Remove Program > Windows Components
Select Application Server … Click Details …
Select Internet Information Services Manager … Click Details …
here you will find SMTP Services …
You can uninstall it easily, but for installing it you will be required Windows Server CD.
One additional cause for this we found was if an account is not set to inherit permissions on the security tab, the user would get this error when trying to access OWA. Once we set permissions to inherit on the AD account they could access their OWA.