Hi,
try
$c=new xmlrpc_client("/", "www.host.ac.at", 443);
instead. But that might not be the problem. That curl error in most cases means that curl couldn't validate the SSL certifcate. This problem can be solved in most cases with:
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
In your case the following modification might solve the problem:
$f=new xmlrpcmsg('checkusr_xmlrpc',
array(
new xmlrpcval($user, "string"),
new xmlrpcval($pwd, "string")
));
$c=new xmlrpc_client("/", "www.host.ac.at", 443);
$c->setCredentials ("fkf", "XXXXX");
$c->setSSLVerifyPeer(0);
$r=$c->send($f, 0, "https");
(if you're using phpxmlrpc available from sourceforge.net)
Thomas