Add media does not work

I am testing your plugin and encountered a problem on my side.
When I click Add media on creating/editing a testimonial I get the normal dialog for selecting an image.
When I click Select the testimonial is displayed again but without the image. No errors whatsoever.
This adding of media works normally on page and post edit.
So what could be wrong?

The page I need help with: [log in to see the link]