Activation problem with membership plugin

We are using the full version (paid for) of your membership plugin.

Everything is working fine apart from one thing. We seem to have the same problem as this chap (and others), he seems to have fixed it but the forum topic doesn't say exactly how.
https://premium.wpmudev.org/forums/topic/members-not-active-beta-version-and-errors-with-query-object-if-connected

As I said, everything is fine but one thing. Members who pay for subscription, after their paypal payment, their account is NOT activated, we have to do it manually. Members are trying to log in
after their payment transaction has been completed but they are not accepted by the system, as their account is not active. As you understand, this can be very frustrating for my clients when there is
no one available to manually activate them, loads of email sent to us and a lot of complaining. Please help.

We have gone through check ups recommended in this and many other posts and also your step by step instructions and everything comes out OK. We have 2 membership levels, free and basic. Free can see the entire site but the basic tagged content. Basic can see everything. Basic is connected
with paypal express with single payments. Payments go through fine and completed but...when we view transactions, there is nothing there, no pending, nothing. And of course, the new member account is NOT activated.

Paypal seems to send IPN replies (or whatever it's called) to this page...http://landscapephotographymagazine.com/paymentreturn/paypalsolo and of course, it just doesn't work. I have attached the results I am getting.

( On another note: I have another website with a payment plugin and it seems to work fine. However, paypal seems to send replies to this page which seems to be part of the plugin? http://dvattika.com/wp-content/plugins/paid-downloads/paypal_ipn.php)

We checked our database and everything seems fine.

We opened subscriptions on Sep 1st and already have a headache.

Please help, we are desperate. I can allow you access to the site if you wish to check anything.

Dimitri Vasiliou