It appears like there is a mixed content warning when I look in the console:
Mixed Content: The page at ‘https://info.ub.hu-berlin.de/annualreports/2014/wp-admin/post-new.php?post_type=ai_galleries’ was loaded over HTTPS, but requested an insecure image ‘http://info.ub.hu-berlin.de/annualreports/2014/wp-content/uploads/sites/4/2015/09/gallica.jpg’. This content should also be served over HTTPS.
Can you change the media to be served over HTTPS? Or disable the SSL on this page to see what happens?