Tell PMPro to validate all PayPal IPN requests. This should only be used as a temporary measure for testing or to keep PMPro running while debugging IPN issues.

pmpro_ipn_validate.php

/*
  Tell PMPro to validate all PayPal IPN requests. 
  This should only be used as a temporary measure for testing or 
  to keep PMPro running while debugging IPN issues.
  
  Add this code to your active theme's functions.php
  or a custom WordPress plugin.
*/
add_filter('pmpro_ipn_validate', '__return_true');