Users not getting assigned to any subscription

I’m having a problem with the Membership plugin. The sign-up and payment process have worked before and the new users got assigned to a subscription after the payment was completed. This has now somehow stopped working. My guess is that it stopped working when we did one of the updates of the plugin. The current version of the plugin we use is 3.5.2.7.

The payment with Paypal (PayPal Single Payments Gateway) still works but when the payment process is finished the user doesn't get assigned to any subscription. I’ve set up the IPN listener in the Paypal account.

Any ideas on what might be the problem here?

Thanks in advance!