We experienced this issue appearing in August 2020 as well. Looks like PayPal changed the Button API and no longer honors the legacy "return" URL: https://www.paypal-community.com/t5/REST-APIs/SmartButtons-Return-URL-s-not-working/td-p/1793386# It seems an upgrade to the JavaScript-rendered "SmartButtons" is required: https://developer.paypal.com/docs/checkout/reference/upgrade-integration/
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