The error is because they use `eval()` in their JavaScript, which browsers block. So far, they have been unable to fix the issue.
If you are a PayPal merchant using a payment button who sells digital goods, this might be why your payments stopped over a week ago. PayPal support is mostly unhelpful.
A workaround purported in the community forum is to start collecting a shipping address, and set the shipping rate to 0.00.
[1]: https://www.paypal-community.com/t5/PayPal-Payments-Standard/Complete-Purchase-Button-failing/td-p/3073625