[till]

You can try to pass explicitly a soap version in the constructor. e.g.:

$client = new SoapClient(null, array('location' => $soap_location,
                                     'uri'      => $soap_uri,
				     'trace' => 1,
				      'exceptions' => 1,
                                      'soap_version'   => SOAP_1_2));

or

$client = new SoapClient(null, array('location' => $soap_location,
                                     'uri'      => $soap_uri,
				     'trace' => 1,
				      'exceptions' => 1,
                                      'soap_version'   => SOAP_1_1));