background image doesn't appear

Hello guys,

I’m using the theme black label and have a problem with the background images. When I set a background image on a page, there is no problem with the upload. On the backend the uploaded image is shown as a thumbnail and I insert the image into post. But on the website itself all you can see is the loading circle.

https://hendrikrojas.meinsign.de/?page_id=2

I reinstalled wordpress several times and tried also different versions (3.6.1 as well). The theme’s developer said it might be a problem with the installation.
What do you think?

Thanks so far.