If you use WordPress multisite and can’t log in to subsites, and get the error “Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.” you can paste a small code in your wp-config.php file:
The location of wp-config.php is in the root of your WordPress install. You can use FTP to download that file, and then uploading it again after adding the line. You can also get into the file manager in cPanel to do the changes here.
The error can occur e.g. if you use domain mapping.