I’m using WordPress 3.5 and I also had the same problem. I read somewhere that adding:
define('CONCATENATE_SCRIPTS', false );
to the wp-config.php file will solve the problem. That fixed my issue.
The line has to be before:
require_once(ABSPATH . 'wp-settings.php');
Mine is after:
‘define(‘WP_DEBUG’, false);’
Hope this help!