I'm getting the the following error when trying to verify payments using the IPN. The same code was tested on the sandbox (before and after the error) and is verifying properly.
<HTML>
<HEAD>
    <TITLE>Access Denied</TITLE>
</HEAD>
<BODY>
    <H1>Access Denied</H1>
    You don't have permission to access "https://www.paypal.com/cgi-bin/webscr" on this server.<P>
    Reference #18.........    
</BODY>
</HTML>
The code I'm using is as follows:
$raw_post_data = file_get_contents('php://input');
pplog("Processing POSTed data");
$raw_post_array = explode('&', $raw_post_data);
$myPost = array();
foreach ($raw_post_array as $keyval) {
    $keyval = explode ('=', $keyval);
    if (count($keyval) == 2)
        $myPost[$keyval[0]] = urldecode($keyval[1]);
}
// read the IPN message sent from PayPal and prepend 'cmd=_notify-validate'
$req = 'cmd=_notify-validate';
if(function_exists('get_magic_quotes_gpc')) {
    $get_magic_quotes_exists = true;
}
foreach ($myPost as $key => $value) {
    if($get_magic_quotes_exists == true && get_magic_quotes_gpc() == 1) {
        $value = urlencode(stripslashes($value));
    } else {
         $value = urlencode($value);
    }
     $req .= "&$key=$value";
}
$ch = curl_init("https://www.paypal.com/cgi-bin/webscr");
curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $req);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
curl_setopt($ch, CURLOPT_FORBID_REUSE, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Connection: Close'));
if(!($res = curl_exec($ch)) ) {
    error_log("Got " . curl_error($ch) . " when processing IPN data");
    curl_close($ch);
    exit;
}
curl_close($ch);
log($res);
I'm on Ubuntu 14.04, with openssl, curl and phpcurl installed, and four hours of debugging.