I just fixed a similar issue. The instructions within tinymce are to change the document.domain value in tiny_mce_popup.js. However this doesn’t do anything because the problem is that tiny_mce_popup.js cannot be accessed or executed.
Instead add this to the top of the tiny_mce.js file:
document.domain = ‘yourdomain.com’; // no need to add www, etc.
This worked for me.