Change Admin URL

change-admin-url.php

<?php
/**
 * Change Admin URL
 * 
 * Copyright (C) 2010  hakre <http://hakre.wordpress.com/>
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 * 
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 * 
 * USAGE: 
 * 
 * Copy the file into  wp-content/mu-plugins  directory and add the 
 * following RewriteRule to your apache configuration or .htaccess:
 * 
 *  RewriteRule ^admin/(.*)$ wp-admin/$1 [QSA,L]
 * 
 * It will rewrite the wordpress admin-URL 
 *  
 *   from: http://example.com/wp-admin/ ...
 *   to  : http://example.com/admin/ ...
 * 
 * @author hakre <http://hakre.wordpress.com>
 * @see http://wordpress.stackexchange.com/questions/4037/how-to-redirect-rewrite-all-wp-login-requests/4063
 * @todo mod_rewrite_rules - filter to insert into .htacces on plugin activation
 */

return ChangeAdminUrlPlugin::bootstrap();

class ChangeAdminUrlPlugin {
	private $renameFrom = 'wp-admin';
	private $renameTo = 'admin';
	static $instance;
	static public function bootstrap() {
		null === self::$instance
			&& self::$instance = new self()
			;
		return self::$instance;
	}
	private function setCookiePath() {
		defined('SITECOOKIEPATH') || define('SITECOOKIEPATH', preg_replace('|https?://[^/]+|i', '', get_option('siteurl') . '/' ) );
		defined('ADMIN_COOKIE_PATH') || define('ADMIN_COOKIE_PATH', SITECOOKIEPATH . $this->renameTo);
	}
	public function __construct() {
		$this->setCookiePath();
		add_action('init', array($this, 'init')) ;
	}
	public function init() {
		add_filter('admin_url', array($this, 'admin_url'), 10, 3);
	}
	public function admin_url($url, $path, $blog_id) {
		$renameFrom = $this->renameFrom;
		$renameTo = $this->renameTo;
		$scheme = 'admin';
		$find = get_site_url($blog_id, $renameFrom.'/', $scheme);
		$replace = get_site_url($blog_id, $renameTo.'/', $scheme);
		(0 === strpos($url, $find))
			&& $url = $replace.substr($url, strlen($find))
			;
		return $url;
	}
}

#EOF;

Gator good find. Your solution makes sense, but it didn't seem to fix it. It looks like the url's are changed, but it's causing admin-ajax to search in the new admin_url which doesn't exist as a folder. Is your solution still working for you? I feel like it might be more of an htaccess issue now.