I have almost the same issue. Everything goes well when I create a new gallery, everything goes well when uploading pictures,
Then it goes not that well : no problem to display thumbs, but no way to show the "mother pictures". I found out why. The problem is linked to file permissions.
Gallery, GalleryTest and Thumbs have required file persmissions
Thumbs pictures have required file permissions
GalleryTest pictures don't have the required permissions and cannot be readed to be displayed.
Therefore, why don't the picture files get the required permissions ? How is this handled within WordPress ?
Someone can help me to understand the process ?