My only guess about this problem is it may be is a timeout issue. Not 100% sure but I think when it happens is after a period of not using the media feature and my server has powered down the drives that contain the media files. When you first browse to a folder and the server has to start up the drives this can delay things for several seconds and it seems to be when this error appears. As I said once it fails you can go back to the same folder and it works.

Hope that helps.