Hi,
When I tried your page, I didn't see the error. I think that the thumbnails are generated now.
Your images are quite big (12 052 992 pixels) so I think that, during the image generation, it could have some memory problem.
But, each time you would render the page, more thumbnails would be generated and after several passes, all thumbnails should be ok.
We would make some tests on the helper image but we use the "PHP GD" library so we cannot manage the memory used during the image resizing.
Regards,