Not sure if this was caused by a change to the API or to our PayPal account, but we were recently getting Fatal Errors on all PayPal Express Checkout transactions on our website. Our cart provider ended up solving the issue. PayPal all of a sudden started wanting an InvoiceID to be submitted with the transaction and it was erroring because our cart was not providing that info. They made an edit to the cart's PayPal Express Checkout module so that it supplies an invoice ID and now all is well. I did speak with several people in different areas of PayPal (including Merchant Technical Support) and none seemed to know how to troubleshoot the issue so I am posting this here so that other people can find the solution before they spend hours in the hold queue.
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