If WordPress? MU Domain Mapping plugin installed, and user mapping domain for him/her blog, in Dashboard the Category, Tags create, remove, edit not work, if the user is use the mapping domain (blogdomain.tld/wp-admin).
In the /wp-includes/script-loader.php file, line 107 the 'url' => admin_url('admin-ajax.php') is write to the html this: http://blog.domain.tld/wp-admin/admin-ajax.php
But the AJAX create, edit work only, if it appears that the output: http://domain.tld/wp-admin/admin-ajax.php (in the http://domain.tld//wp-admin/category.php link).