Hi, I ran a test on your website and I can reproduce the error. When the error is returned PayPal is not called at all but we are on your server/website on this page:
/payment/ps_paypal_pro.php?mode=express&payment_id=20&x=37&y=15
Also in our logs for your account I can't find any error or call towards PayPal. I guess something in your module/plugin is not calling PayPal correctly. Also a thing I noticed is that the page called is ps_paypal_pro.php that looks like you are calling PayPal PRO methods but your acccount is not PRO so will not work with PayPal PRO methods.
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