[Ken Case]

I believe you might have to turn off "Block images and other inline content from third-party sites" from the site which uses paypal in its checkout. (If I remember correctly, the checkout process tries to load some code from paypal to let it know about the purchase, but that ad-blocking setting prevents it from reaching paypal so it gets confused when you actually go to checkout.)

You might give that a try and let us know whether it resolves the issue for you!