Edit: Payments are again working. Thankyou for your patience.
Paypal automated payments are currently not being accepted by Paypal
We are working with Paypal to figure out why not and we will keep this post updated with details.
The error page that is shown to the user recommends that the user request a manual invoice from the website owner. They can do this via pressing the "Invoice me!" button. This button is normally there for users who do not have a Paypal account and do not want to create one.
Once the user has clicked this button, you will receive a notification requesting that you send a manual Paypal invoice to this user.