Unable to verify $_POST vars – What to check?

I was hoping someone could tell me which things i need to check to hopefully find the cause of this problem, since as someone working on a friend’s site I’m new to WordPress and s2Member. I ran into the following problem when attempting a paypal subscription:

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. If you are the site owner, and you're absolutely SURE that your configuration is valid, you may want to run some tests on your server, just to be sure $_POST variables are populated, and that your server is able to connect/communicate with your Payment Gateway over an HTTPS connection.

s2Member uses the WP_Http class for remote connections; which will try to use cURL first, and then fall back on the FOPEN method when cURL is not available. On a Windows® server, you may have to disable your cURL extension; and instead, set allow_url_fopen = yes in your php.ini file. The cURL extension (usually) does NOT support SSL connections on a Windows® server.

Please see this thread for details regarding the ideal server configuration for s2Member

I did search arround to see what i could find about this, and based on that tried to gather the kind of information needed to debug this kind of thing:

• I ran the server scanner, aside from (possibly) increasing the memory allowance, everything else seemed to be good
• managed to find my way into the logs and grab some details from ‘paypal-rtn.log’
• grabbed a copy of the source on my Paypal button (in case it’s an invalid custom variable or something)

The Log:

PHP v5.3.23 :: WordPress® v3.4.2 :: s2Member® v130123 :: s2Member® Pro v130123
Memory 25.60 MB :: Real Memory 25.75 MB :: Peak Memory 25.69 MB :: Real Peak Memory 25.75 MB
www.beercollector.com.au/?s2member_paypal_return=1&s2member_paypal_return_tra=fnIyOnRXdk1XUFRodWFCMW8zMnI5ZnFpRTJHWUtKb29XUnRqOjIyM2VjNTU1MzA2YTcxOWEzNDk3NWQyZDM1MjcxYjYwfBdgefKG0LOyR_YqrV_ZiVhV4hQcgsaw6A86LuKrT1ih9hjlkTaaXK0APlDzAfcrA59F2FXi4uZW16m0ZEAcbn_SUix9X8XYSpJSIyAt6X12ByWzVCtwRWOktyi9bgSrVmBn7Lkpizww6HkdGZR8x-_usoPIL6W2n2tKEXnhc4Ftr-oBnJU5mFJYuKbSOr5leXbaNpnQg8VJH39E17yYh-i8_gDAuKQlLsflKJXJoCXdPxAfytngnnn4x_WZbmsjuiJMbeNtRSnKjCb2i3bQB5f-FK80xF9rdZj45xrX2QBAYwtkHj1H675ht9YTwdtLVszXPKxICRihPd0mIXhbhON3BRXtQtiuQAPbKMz7IxUb4o3Bv4vop9tGTZy37m9GFw&tx=9X1414810L0487929&st=Completed&amt=1.00&cc=AUD&cm=www%2ebeercollector%2ecom%2eau&item_number=&sig=l9QYRJVjb6eQwJ6RBT%2f0%2fK7rtI5KdzzHaE5F0OFRh1mrnLv9xJzNl40gEP9i%2fTPAA0mtxOwqF2pnKBn8Q6ujhmUqfyqZ1oTgWeb%2fnG48HohfLRYLomc1GG0GuMPuUVgm7L%2f9MjsrRbQPpl2QXSy%2fcMgWNPvBmAeR5OISPa7wBNI%3d
User-Agent: Opera/9.80 (Windows NT 5.1; U; en) Presto/2.7.62 Version/11.01
array (
  's2member_log' => 
  array (
    0 => 'Unable to verify $_POST vars. This is most likely related to an invalid configuration of s2Member, or a problem with server compatibility.',
    1 => 'If you're absolutely SURE that your configuration is valid, you may want to run some tests on your server, just to be sure $_POST variables are populated, and that your server is able to connect/communicate with your Payment Gateway over an HTTPS connection.',
    2 => 's2Member uses the `WP_Http` class for remote connections; which will try to use `cURL` first, and then fall back on the `FOPEN` method when `cURL` is not available. On a Windows® server, you may have to disable your `cURL` extension; and instead, set `allow_url_fopen = yes` in your php.ini file. The `cURL` extension (usually) does NOT support SSL connections on a Windows® server.',
    3 => 'Please see this thread: `http://www.s2member.com/forums/topic/ideal-server-configuration-for-s2member/` for details regarding the ideal server configuration for s2Member.',
    4 => 'array (
  's2member_paypal_return' => '1',
  's2member_paypal_return_tra' => 'fnIyOnRXdk1XUFRodWFCMW8zMnI5ZnFpRTJHWUtKb29XUnRqOjIyM2VjNTU1MzA2YTcxOWEzNDk3NWQyZDM1MjcxYjYwfBdgefKG0LOyR_YqrV_ZiVhV4hQcgsaw6A86LuKrT1ih9hjlkTaaXK0APlDzAfcrA59F2FXi4uZW16m0ZEAcbn_SUix9X8XYSpJSIyAt6X12ByWzVCtwRWOktyi9bgSrVmBn7Lkpizww6HkdGZR8x-_usoPIL6W2n2tKEXnhc4Ftr-oBnJU5mFJYuKbSOr5leXbaNpnQg8VJH39E17yYh-i8_gDAuKQlLsflKJXJoCXdPxAfytngnnn4x_WZbmsjuiJMbeNtRSnKjCb2i3bQB5f-FK80xF9rdZj45xrX2QBAYwtkHj1H675ht9YTwdtLVszXPKxICRihPd0mIXhbhON3BRXtQtiuQAPbKMz7IxUb4o3Bv4vop9tGTZy37m9GFw',
  'tx' => '9X1414810L0487929',
  'st' => 'Completed',
  'amt' => '1.00',
  'cc' => 'AUD',
  'cm' => 'www.beercollector.com.au',
  'item_number' => '',
  'sig' => 'l9QYRJVjb6eQwJ6RBT/0/K7rtI5KdzzHaE5F0OFRh1mrnLv9xJzNl40gEP9i/TPAA0mtxOwqF2pnKBn8Q6ujhmUqfyqZ1oTgWeb/nG48HohfLRYLomc1GG0GuMPuUVgm7L/9MjsrRbQPpl2QXSy/cMgWNPvBmAeR5OISPa7wBNI=',
)',
    5 => 'Redirecting Customer to the Home Page, due to an error that occurred.',
  ),
  's2member_paypal_return_tra' => 'fnIyOnRXdk1XUFRodWFCMW8zMnI5ZnFpRTJHWUtKb29XUnRqOjIyM2VjNTU1MzA2YTcxOWEzNDk3NWQyZDM1MjcxYjYwfBdgefKG0LOyR_YqrV_ZiVhV4hQcgsaw6A86LuKrT1ih9hjlkTaaXK0APlDzAfcrA59F2FXi4uZW16m0ZEAcbn_SUix9X8XYSpJSIyAt6X12ByWzVCtwRWOktyi9bgSrVmBn7Lkpizww6HkdGZR8x-_usoPIL6W2n2tKEXnhc4Ftr-oBnJU5mFJYuKbSOr5leXbaNpnQg8VJH39E17yYh-i8_gDAuKQlLsflKJXJoCXdPxAfytngnnn4x_WZbmsjuiJMbeNtRSnKjCb2i3bQB5f-FK80xF9rdZj45xrX2QBAYwtkHj1H675ht9YTwdtLVszXPKxICRihPd0mIXhbhON3BRXtQtiuQAPbKMz7IxUb4o3Bv4vop9tGTZy37m9GFw',
)

The Form:

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
 
 
 <!-- Instant Payment Notification &amp; Return Page Details -->
 
 
 
 
 <!-- Configures Basic Checkout Fields -->
 
 
 
 
 
 
 
 
 
 <!-- Configures s2Member's Unique Invoice ID/Code  -->
 
 <!-- Identifies/Updates An Existing User/Member ( when/if applicable )  -->
 
 
 <!-- Identifies The Customer's IP Address For Tracking -->
 
 
 <!-- Controls Modify Behavior At PayPal® Checkout -->
 
 <!-- Customizes Prices, Payments &amp; Billing Cycle -->
 &lt;!----&gt;
 
 
 
 &lt;!----&gt;
 &lt;!----&gt;
 &lt;!----&gt;
 
 
 
 <!-- Displays The PayPal® Image Button -->
 
</form>

I Hope that’s enough. Even if you don’t know exactly what the problem is, any advice on what to check and where would be very helpful