No large Images in Lightbox

After Upgrading to 3.5.6 the Lightbox feature in the Customer Galleries istn´t working. It seems, like the images are not processed by the Image.php script.

The Lightbox loads infinitely and nothing happens. You can see it here:

http://www.fotogeschaeft-am-frauentor.de/galleries/katja-alvaro-mit-mika/

ID: 2015-083
Password: mika

Even when i´m call the URL to the large image directly there is no image:

http://www.fotogeschaeft-am-frauentor.de/wp-content/plugins/image-store/image.php?i=fI2aoGuilKJ1

But the large file ist uploaded correctly and is available on the server:

http://fotogeschaeft-am-frauentor.de/wp-content/_imsgalleries/gallery-24811/DSC_0120.jpg

Is this an access rights problem because of the password protected galleries or have you an idea, what is going wrong?

If you need an admin account to the page, please let me know and give me your email to set the account up. It is really urgent, because this is a productive live system and my customer needs a working gallery

Thank you

Markus