This is an error I've seen pop up on various themes and installations with no connected cause. There are tons of threads on this spread out over the wordpress forums and other support forums and the fixes are never quite the same.
Try #6 and look at #7 here: http://wordpress.org/support/topic/25-imagemedia-uploader-problems
Additionally, try changing the permissions on your wp-content/uploads folder to 755
In your Settings>Media try checking/unchecking the upload organization option and save the changes. See if either setting fixes the issue.
If you can find out what version of php you are running on your server, that may give a clue as well.
If nothing else, call your hosting provider and explain the issue. They might have run into it before and changing a setting in their modsecurity files might do it.
Its a lot of suggestions, but as I said there are a lot various little things that can all cause the same symptom.