I am facing the same issue. I am using Woocommerce with woocommerce paypal express plugin. when I select to log in the paypal account I see "return to merchant" and when I use pay via the card I see session expired try again page. When I tried a paypal partner company Angell EYE in woocommerce, I saw the same error but when I contacted Angell EYE guys they added their sandbox API details in my woocommerce and with that I was able to complete my order process. But again when I entered my API details its showing the two different issues: When paypal login >> it shows>> "RETURN TO MERCHANT" When use card >> It shows>> "Session expired please try again (old paypal page)" Check the screenshot. https://prnt.sc/i4ikhw http://prntscr.com/i4il9p http://prntscr.com/i4ilgj http://prntscr.com/i4ilo8 Any solution.
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