Downloaded file is empty

Hello.

I’m running a site where I’m going to sell pictures.
I’m using the download option in wp e-commerce only for this.
The problem is that when someone has bought a picture and downloads it, they get an empty file with the correct name. I’m using WordPress version 4.0 and wp e-commerce version 3.8.14.1.
The site is: http://www.bildbanken.photos/

Thank you for your answer in advance.

https://ww.wp.xz.cn/plugins/wp-e-commerce/