Problem with: XML-RPC server accepts POST requests only.

Yes, but I used only POST requests and it keeps replying that it only accepts POST requests.
The code I used uses POST requests...I know it for sure because connecting to xmlrpc.php with the same code worked (and also because it's the same code you have on your Handbook)

Any idea why it replies in that way?
I looked at the XMLRPC Server module code and I saw that it includes drupal core's xmlrpc files, but somehow they trigger this error when they are included in this module and not when they are included in xmlrpc.php

I see.
Sorry for the critical...it is critical for me, not for Services XD

So...let's see, this is the code I'm using:

function assemble_request($method, $args = array(), $dump_request = FALSE){
        $request = xmlrpc_encode_request($method,$args);
        $context = stream_context_create(array('http' => array(
                'method' => "POST",
                'header' => "Content-Type: text/xml",
                'content' => $request
        )));
        if($dump_request){
                echo '<pre>';
                echo htmlentities($request);
                echo '</pre>';
        }
        return $context;
}

function dump_response($response){
       echo '<pre>';
                if (xmlrpc_is_fault($response)) {
                        trigger_error("xmlrpc: $response[faultString] ($response[faultCode])");
                } else {
                        print_r($response);
                }
       echo '</pre>';
}

function make_args_array($method, $session, $args = array(), $dump_args = FALSE){
        $domain = 'http://test.rel.to';
        $timestamp = (string) time();
        $nonce = user_password();
        $key = '018a73f2411cb8ca8fcf69541fd2ebb1';
        $hash = hash_hmac('sha256', $timestamp .';'.$domain .';'. $nonce .';'.$method, $key);
        $arrHash = array();
        array_push($arrHash, $hash, $domain, $timestamp, $nonce, $session);
        if($dump_args){
                echo '<pre>';
                var_dump(array_merge($arrHash,$args));
                echo '</pre>';
        }
        return array_merge($arrHash,$args);
}

function user_password($length = 10) {
        $allowable_characters = 'abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ23456789';
        $len = strlen($allowable_characters) - 1;
        $pass = '';
        for ($i = 0; $i < $length; $i++) {
                $pass .= $allowable_characters[mt_rand(0, $len)];
        }
        return $pass;
}

$ctxtSysConnect = assemble_request("system.connect", NULL,TRUE);
$file = file_get_contents("http://test.guildofmessengers.com/services/xmlrpc", false, $ctxtSysConnect);
print $file; //let's see what the server really returns
$response = xmlrpc_decode($file);
dump_response($response);
$session = $response['sessid'];

$arrArgs = make_args_array('user.login',$session,array("username","password"));
$ctxtSysConnect = assemble_request('user.login', $arrArgs,TRUE);
$file = file_get_contents("http://test.guildofmessengers.com/services/xmlrpc", false, $ctxtSysConnect);
print $file; //let's see what the server really returns
$response = xmlrpc_decode($file);
dump_response($response);
$session = $response['sessid'];

$arrArgs = make_args_array('user.logout',$session);
$ctxtSysConnect = assemble_request('user.logout', $arrArgs,TRUE);
$file = file_get_contents("http://test.guildofmessengers.com/services/xmlrpc", false, $ctxtSysConnect);
print $file; //let's see what the server really returns
$response = xmlrpc_decode($file);
dump_response($response);

Server always replies with "XML-RPC server accepts POST requests only."

in /includes/xmlrpcs.inc, the check that triggers the error is: (at line 56)

$data = file_get_contents('php://input');
  if (!$data) {
    die('XML-RPC server accepts POST requests only.');
  }

but it doesn't make any bell ring in my head :/

EDIT: I also used another Drupal installation to connect. Using xmlrpc() gives the same results

I was finally able to track the problem.
ALWAYS use a language prefix in the URL pointing to the server...otherwise drupal will try to guess which language it should use and if it finds one it will redirect. And when you get redirected all the POST content is lost.
I had this problem amplified because my website uses /en/ as language prefix for English which is the default language...so if you don't use a language prefix you will always get redirected.