lightbox not centered

The plugin is great. However, I have the problem, that the lightbox is not centered when opened. It always is very close to the bottom. I wonder, if someone could have a look at my site and maybe have an idea, what the problem is.

http://muenchenblockd.whitelabel-muenchen.de/category/galerien/

http://wordpress.org/extend/plugins/jquery-colorbox/