Hi Jason,
I found this error 500 in the log file :
2014-01-05 22:39:23 POST /Redirect.asp |989|80040e14|Number_of_query_values_and_destination_fields_are_not_the_same. - 178.118.226.183 HTTP/1.1 Mozilla/5.0+(Windows+NT+6.3;+WOW64)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/31.0.1650.63+Safari/537.36 http://www.stickers.be/Checkout_PayPal.asp 500 1404