Personally I think Paypal should put a real error message instead of this catch all error message. I figured out the problem for myself. I was trying to test payments and PDT returns so I set the form to submit to https://www.sandbox.paypal.com/cgi-bin/webscr but the "business" paramater was set to my real business email. I didn't realize that I needed to set the "business" paramater to my sandbox account business test email. After I changed that, it worked without errors.
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