This problem only occurs when an edit dialog with a richtext field is displayed... and on IE9 (running in IE9 standards mode).
When creating a link in a text (by clicking the link icon), an empty popup is displayed. None of the buttons can be clicked and the popup cannot be closed.
See also the enclosed screenshot!
The problem (and a possible solution) is described on the following site: http://www.tothepointsolution.com/site-blog/article/bug-fix-for-fckeditor-quotregisterdollarfunctionquot