Photoswipe captions are not showing up

Today I purchased the photoswipe add-on. It works, but I can not get it to show the captions of the images in the lightbox.
See this example page:
https://dev.marres.org/en/echt-een-engelse-titel-maken/

The images do have a caption but it does not show when you click on them for the lightbox.

What should I do to make them appear?

thank you,