I am having this problem as well. I decided to comment out line 141 and the autoresponder may be working (it seems to be flaky).
Now i get this error:
[10-Dec-2013 14:59:11 America/Denver] PHP Fatal error: Call to undefined function get_option() in …/wp-content/plugins/paypal-responder/ipn.php on line 145
IN paypal IPN history there is a status code of 500. If i type the listener url into a browser i do not get a 500. I called my host and they say it is paypal.
PLease respond!