ERROR: Unable to verify $_POST vars.

Hello.
I installed the free version of s2member and was able to
– go to my sandbox paypal seller account
– pay with my sandbox paypal buyer account
– receive acknowledgement of payment

but then I get the message:
ERROR: Unable to verify $_POST vars.
Please contact Support for assistance.
This is most likely related to an invalid configuration of s2Member, or a problem with server compatibility.

I checked my paypal settings with s2member-server-check-120526.php and everyhing looked fine.
I added php.ini in the root directory and added allow_url_fopen = On
I checked my server settings with fopen-test-https-paypal.php and curl-test-https-paypal.php and the test failed for both.

From another thread I understand that it is an issue with the server not allowing https urls. What exact words do I use to explain what I need to my provider?

Thanks in advance!
Manuela