ckeditor open dialog

CKEditor - [CKEDITOR.dialog.openDialog] Dialog "link" failed when loading definition

I think this happened to me some weeks ago but I am blogging about it only now (the last month or so has been overly hectic, so no time for "exotic" activities like blogging).

In an ASP.NET website installation that has been running perfectly for months and using CKEditor for wysiwyg content entry, suddenly the users started reporting they can no longer click the "Link" button in the editor toolbar to bring up the Link dialog for turning the selected text to a hyperlink.