When WooCommerce automatically adds notes to order, they are treated like comments. attribution-wordpress.php:548 then tries to set a cookie for that “user”.

error": {
    "errno": 2,
    "errstr": "Cannot modify header information - headers already sent",
    "errfile": ".../wp-content/plugins/attribution/includes/class.attribution-cookie.php",
    "errline": 26
}