This means that the call to mysqli_connect() failed to make a connection to the server, which in turn made the call to mysqli_errno() fail as it was not given a valid resource.
Try changing the code inside dml.mysqli.class.inc to the following and try again.
function getErrorNo ()
// return number of last error.
{
if (empty($this->dbconnect)) {
$errno = null;
} else {
$errno = mysqli_errno($this->dbconnect);
} // if
return $errno;
} // getErrorNo
Hopefully this should display the reason for the failed connection.