While using a shadowbox there is only a black screen

Only a black sreen if I open a presentation in a shadowbox!
I use Joomla and I want to open a presentation in a shadowbox. But all what happened is a black sreen in the box if I open it in fullscreen mode.
Otherwise (without fullscreen) there is the prezi page and the presentation is not available. There is only a white "nothing" in it.
The plugin (shadowbox for Joomla) is working fine without problems. Only the Prezi presentation does not work