Hello, I am experiencing the exact same problem. Since a couple of weeks ago, around the beginning of August 2018, when making a test payment to my Sandbox merchant account, the return URL with PDT enabled is no longer working. Instead of being redirected to the return URL, paypal redirects to a generic Checkout page with the following URL: https://www.sandbox.paypal.com/webapps/hermes?flow=1-P&ulReturn=true&token=1AS676027T527961S&useraction=commit&mfid=1534918923162_a04e73c069f34&country.x=DE&locale.x=de_DE#/checkout/genericError?code=PAYMENT_ALREADY_DONE The page just shows the PayPal banner graphic and the following text: Your payment was completed. To continue shopping, please return to the merchant. And the page title is: PayPal Checkout - Please try again. The IPN works as expected and I receive the confirmation email sent by the IPN Listener I have setup. The above page is curious because the title says to please try again, however the main text says payment completed. And in the URL, there is a generic error code with "Payment_already_done". In any case, even when I disable PDT and try a different return URL, I still get redirected to the generic PayPal Checkout page. I even tried to create a new merchant Sandbox account but I still have the same problem.
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