The following error sometimes presents itself when trying to login to the subsite of a WordPress multisite.
ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.
Add the following to your wp-config.php or user-configs.php if you’re on GridPane
define('ADMIN_COOKIE_PATH', '/');
define('COOKIE_DOMAIN', '');
define('COOKIEPATH', '');
define('SITECOOKIEPATH', '');
Some people have reported that the following will also work
define( 'COOKIE_DOMAIN', $_SERVER['HTTP_HOST'] );