I can confirm I’m getting the same error. With the Piklist plugin activated on a WP3.5 install, the media library reports an unspecified error when attempting to upload an image, although it seems that the images are uploaded. Because with the Piklist plugin subsequently deactivated, the images appear in the media library and the upload functions normally.

Furthermore: with Piklist active, the screen fails to refresh when attempting to add a new Custom Field, or a Category, to a Post. Again, the additions are apparently made in the background and a manual refresh of the ‘edit post’ screen will display them.

Perhaps the two issues are related?