I ran into this the other day. I thought possibly something was broken on the OWA servers, but after reviewing, I didn't find anything wrong with OWA.
Amazingly, what I found was that the site url had been changed and the link to the site notebook stays the same! Attempting to click on the link will cause OWA to attempt to open the site notebook, but it will fail miserably. You will need to update your navigation node to point to your new site url.