I'm also having trouble with this!
I assume this is a bug and not a feature since it displays an error message rather than a "nice" user friendly message.
We have a moderator role that should be able to view the grades (no problem here), feedback comments (again, fine) and feedback files (sees the filenames and links, but once the user clicks on a link, it receives an error).
Any help would be greatly appreciated.
Thanks
Gareth