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Try uncommenting some of the debug output functions in ipn.php and checking your error logs to see where and why it fails.

According to the ipn.php file (line 108) your primary PayPal email is compared to the email you have in your easyfileshop settings. If you’re getting the error it means they differ (i.e. if you have specified a secondary PayPal email address in easysfileshop config instead of the primary one).
You can modify your ipn.php file to print both values to the error log and see why do they differ.