wp User Frontend pay per post not working

We are trying to use the free version as pay per post. It gets as far as Paypal taking the money, but the posts stay as pending, with no entry in the transaction table, and investigation shows the validateIPN function returning false.
At the Paypal end we have set it to use IPN and return to the website, payment page, payment success or home page and it still falls at the IPN stage.

It works perfectly until we use pay per post.

Should it work on this, and there any other settings we need to use?

https://www.ads-software.com/plugins/wp-user-frontend/