Unknown error caused by Ajax while using colorbox

I installed on D7 the module Colorbox. I created a new content type which has an image field, from which the imagedisplay is directed by Colorbox.

When I create a new node of this content type and try to upload an image into this field I get the following Ajax-error (see image).

Are my settings wrong, is it a bug, or is it something else? Who can help me?