Paypal not working in Protected Content

Hi there,
I think I need some help configuring Paypal with Protected content.
Somehow the payments made with paypal aren't communicated back the website.
The IPN gets a 404 error when I look at the history in paypall.
Where does it go wrong?
I tried both the "paypal-standard" and the "paypalstandard" variations of the IPN URL.

Hope to hear from you.

Kind regards,
Bart Jan