Paypal not returning data

Hello,
I was hoping this would work out-of-the-box, but am having lots of problems with the data returned by PayPal. I’m not an expert in the technical end of things, so am trying to understand what is happening.
I’m currently using sandbox and the process works as it should (I can subscribe, get redirected to PP then back to the Thank You page). However, my thank you page is blank. I.e, [groups_paypal_return] is empty. Also, the user that makes the payment should be automatically subscribed to that subscription group, but that too does not happen.
My test page is here:

Test

Things I’ve tried/checked the following:
– Made sure IPN url is correctly set in my sandbox account.
– Made sure I’m not getting an security certificate error. I’m using https and when I check the notification URL that you specify, I don’t see errors (I get a blank page).
– I added the suggested debugging lines in wp-config.php . Unfortunately there is no error log created in wp-content subdirectory or anywhere else. I even defined the path in WP_DEBUG_LOG, but no log file is ever written. At one point I even set WP_DEBUG_DISPLAY to true and saw one error displayed on the page, but it was unrelated and I fixed it. But I get no other errors.
Your help is most appreciated/needed.