Hi Dan,
I just had the same issue and I fixed it by re-saving all the folder and files on the Media section within Umbraco. You could also checked if you don't have a corrupted file somewhere or if one of the media document type does not have an extra property not really needed (it was the case for me). So I can;t really say why it breaks but doing all these checks could solve your problem if you still have it as your post is quite old!
Ju
Solutions come from this space between the computer and the chair... stay optimistic :) -
www.juliens.org