Solution:1
This is another solution
define('ADMIN_COOKIE_PATH', '/');
define('COOKIE_DOMAIN', '');
define('COOKIEPATH', '');
define('SITECOOKIEPATH', '');
to wp-config.php file.
Solution:2
When changing between subdomain and folder-setup, there might be a error in the wp_site-table. Make sure that the path is set to / for the main domain.