I know I'm late to this topic, but I just ran into this error on a merchant website that I have been using a lot without errors in the past. I think the error is related to the PayPal Auto-Login feature that is used in conjunction with the Express Checkout. This is what I did (I can't guarantee it will resolve the problem for everyone). 1. Log into your PayPal account. 2. Click on the gear icon on top to take you to the settings page. 3. Click on the Security tab on top. 4. Click on the Auto-Login option. 5. You will have a list of servers or IP addresses that you have authorized auto-login. Click on the Turn Off option to remove it. 6. I don't know which IP addresses represent what computers, so I just turned them all off. When I went back to the merchant website to try and pay again, the PayPal webpage needed me to log in manually but the payment transaction went through.
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