I don't know whether this will solve your particular issue, but I solved a similar problem with Paypal when I discovered that, inside my Paypal account (the one Moodle is paying to), I needed to enter a specific web address for the Moodle file that processes Paypal's IPN (Instant Payment Notification).
It may be difficult to find the place in your Paypal account where you enter the IPN address. Here's the path in a recent Paypal (they sometimes change the navigation):
Under your Paypal account (the symbol of a person on your first page), choose Profile and Settings.
Then choose My Selling Tools, and look there for Instant Payment Notification. When you click this, you'll see a place to enter an http address for the Moodle file that handles Paypal IPNs.
Presently, that address looks something like the following (substitute your server and moodle name):
https://yourserver.com/moodle/enrol/paypal/ipn.php
If we fail to re-set this IPN address inside Paypal every time we create a new Moodle, students start having problems getting enrolled through Paypal.
Make sure the Paypal IPN address points to the ipn.php file in an active copy of Moodle.
I hope this helps.
