I had this problem too. Mine had been working since 2016 then suddenly stopped. My problem was that I added the cmd=_notify-validate to the end of the END of the list of parameters instead of at the beginning. From their IPN guide: Create a request that contains exactly the same IPN variables and values in the same order, preceded with cmd=_notify-validate. I changed my code to add the cmd=_notify-validate to the beginning of the list to fix the problem. I hope this helps.
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