Bumping this thread.
Having the exact same issue without finding any solution to this or what’s causing it. Albeit after some inspection i noticed the /index.php?callback=json url can be retrieved with full (and apparently valid) json data when using an Unix command line tool wget.
Also upon inspection with FireBug it does appear the Content-Type is “application/json; charset=UTF-8” which seems correct. But nothing appears in the browser (even if it could tell me the data was 11k in size).
The URL in question is: https://www.radiozeeland.com/?callback=jsonp&api_key=true&format=json&method=gallery&id=8
An example of a correct JSON callback reply should been like: https://photography.christophassauer.de/index.php?callback=json&api_key=true&format=json&method=gallery&id=9
WordPress 3.3.2 + ngg.slideshow.min.js?ver=1.06
