OK, I see this page from paypal.
https://www.paypal-knowledge.com/infocenter/index?page=content&id=FAQ1766&expand=true&locale=en_US
Are you using a SSL on your server? NBS doesn't require this, but if your server/website is using https instead of http then I can understand why the IPN return SSL might need to be upgraded. However I think probably the easy work around would be just to make the IPN return url "<ReturnNotifyURL>" a standard http.
https://www.paypal-knowledge.com/infocenter/index?page=content&id=FAQ1766&expand=true&locale=en_US
Are you using a SSL on your server? NBS doesn't require this, but if your server/website is using https instead of http then I can understand why the IPN return SSL might need to be upgraded. However I think probably the easy work around would be just to make the IPN return url "<ReturnNotifyURL>" a standard http.