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I believe what's being described here is a particular case of CiviCRM issue financial/27 - PayPal Standard doesn't handle IPNs.  The issue mentions recurring payments, but any payment involving an IPN is likely affected.

You can help solve this by testing the proposed fix and reporting on whether it solves your problem, creates new problems, etc.

Note also that this sounds related to the same section of code improved in financial/14 which deals specifically with PayPal Express.  You may want to see if upgrading to Civi 5.3.0 helps matters.