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When $_SERVER[‘HTTP_USER_AGENT’] is not set, most requests will give a php error notice which will for example prevent using the Woocommerce API, or any header redirect.
A work around is to set that key (when unset) or to suppress notices.
It happens here: woocommerce-product-price-based-on-countries/woocommerce-product-price-based-on-countries.php on line 135