Tobias, it might not be the theme, but the other plugins you use on your site. We’re using this on doizens of sites, with very different themes and plugins and it always worked.
I wrote you before – your image links don’t have rel=”lightbox”. It will never work if there’s no selector to recognize if given images is to be opened in lightbox.
How to Get Support
After you register and login to the site, you may post all plugin support questions in the Support Forum.
If you need to provide private info, please create a ticket and then reply to it using Set as private reply option.