anyone getting "IPN warning" emails from PayPal?
Thank you for the confirmation others are seeing this issue. In the end, this solved it for us - add lines 4-6 from the following code (lines 1-3 and 7-9 included for reference) to file /CRM/Core/Payment/PayPalProIPN.php

CRM_Core_Error::debug_var('GET', $_GET, TRUE, TRUE);
CRM_Core_Error::debug_var('POST', $_POST, TRUE, TRUE);
if ($this-&gt;_isPaymentExpress) {
  if (empty($this-&gt;_inputParameters['rp_invoice_id'])) {
    return;
  }
  $this-&gt;handlePaymentExpress();
  return FALSE;
}