In jQuery 1.5+ (bundled with WordPress 3.2+), AJAX requests on sites with IDN domains don』t work in IE or Opera.
On Edit Post screen, adding a new category results in JS error:
Message: 'parsed.responses' - is null or not an object
On Categories screen, adding a new category silently fails.
According to jaubourg』s comment in jQuery bug tracker, the request is considered cross-domain and, since IE doesn』t support cross-domain requests, AJAX won』t find a suitable transport and bail out.
This plugin implements a workaround suggested there by making AJAX requests relative.
WordPress Trac ticket: #18952
