[Plugin: WP Smush.it] Unknown error while processing

Hello,

I just installed wp smushit, and I’m getting this error when trying to smush an image in my media library.

unknown error while processing http://www.my-domain.com/wp-content/uploads/2011/02/my-image.jpg (/var/www/www.my-domain.com/public/wp-content/uploads/2011/02/my-image.jpg)

Anyone else experiencing this?