I'm getting this error as well. After posting your order to PayPal using the standard checkout, the order will process. Upon redirection back to the site I get the following:
PDT Failure: FAIL
Error: 4002
Some of the things I turned up in research was that this is will happen when PayPal displays an "oragne box". I also heard this will occur when you try to send sandbox info to production and vice versa.
I got the error with the orange "Return to <sitename>" button. I'm a bit stumped on how to get around this. Any help/tips/feedback/donations are appreciated.
Thanks.