I've lost half a day on that problem: I was constantly receiving debug emails with this error message and didn't get a clue.
I didn't use the provided filter to change the sslverify param so I really didn't know was the problem was... until I realized that sslverify doesn't default to false in the _validateMessage() function!
After changing the $params line 590 to
$params = array( 'body' => $_POST, 'sslverify' => apply_filters( 'paypal_framework_sslverify', false ), );
The error messages stopped and all IPNs were successfully verified \o/
I love this plugin by the way, great job!