Updated: We have resolved this issue. For reference, though, the original post follows.
We’ve seen errors from instructors and students trying to access files (I’ve seen it in PDFs, HTML documents and Excel files) attached to learner feedback on grades. In all of these cases, the student’s submission and grade history were fine, but if the instructor was responding with an attachment, the student couldn’t see that attached feedback.
We’re working on a permanent fix, but do believe that the problem is intermittent and only happening to lucky users who hit the site via a certain hidden path. So, if you do see this error, you can try opening feedback in another browser or clearing your browsing history and trying again. This “try again” method means that your browser could hit another hidden path, and one that works.