Image and audio upload problem in WP 2.5 RC1

Sometimes when I upload images to a new post I get the following error :

Error #2044: Unhandled IOErrorEvent:. text=Error #2038: File I/O Error.

It’s not on all files and it’s not a permissions issue because other files in the same directory work just fine. The same file fails every time but the image is valid (it can be read by Photoshop as well as other image viewers etc)

The error occurs during the ‘crunching’ stage of images and audio files (although, as I said, not all).