This question specifically deals with the Multiple Post Thumbnails plugin, though the problem may have to do with my installation of WP itself as nobody else seems to have had this problem.
I am using WP 3.0.5, completely stock with the exception of the plugin. As per the instructions, I am initializing the plugin as such:
$video_thumb_secondary = new MultiPostThumbnails(array( 'label' => 'Thumbnail Image', 'id' => 'video-thumbnail', 'post_type' => 'video' ));
The code above is called in a function run during the admin_menu hook.
When attempting to upload/select an image and then clicking on 'Use as thumbnail image' I get the following error: "Could not set that as the thumbnail image. Try a different attachment." I have also tried initializing the plugin with the default 'post_type' of 'post' and have run into the same error.
As I said, I have seen no other documented cases of this error with this plugin, and hardly any at all of this error in general with WP. Hoping somebody can shed some light on it...
** EDIT ** Resolved my own issue. The code above must be run outside of any function in your functions.php file. I naturally assumed that it should be defined in one of the admin menu hooks, but that is not the case.