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