After upgrading to WordPress 3.4 I noticed that most of my Featured Image thumbnails were missing. It seems that thumbnails I had set manually were still displayed, but ones that were generated by this plugin wouldn't show up anymore.
The missing thumbnails are gone from the front end and are no longer set as Featured Image on the Edit Post screen. They are still listed in the gallery but the their preview images are blank.
When creating new posts, this plugin doesn't generate thumbnails automatically anymore.
I looked around in the MYSQL database and noticed that thumbnails generated by this plugin don't have a _wp_attached_file entry in the _postmeta table. Manually set thumbnails, which still work, have _wp_attached_file set to their specific image URL. Maybe this has something to do with it?