QUdpSocket::writeDiagram, results in a NetworkError when using IPv6 link-local addressing (e.g. ff02::1) on Mac OS X.
The specified scope id is definitely correct, and the source code works fine under Windows.
And it only occurs when no dynamically allocated IPv6 address is set to the interface.

Testcase:

QByteArray datagram = "Broadcast message";

QUdpSocket socket (this);

QHostAddress addr ("ff02::1");
addr.setScopeId ("4"); // Scope id 4 is en0 under this system

if (socket.writeDatagram (datagram, addr, 4545) < 0)
{
qDebug () << "Errno: " << socket.error();
qDebug () << "Errmgs: " << socket.errorString();
}

output:

Errno: QAbstractSocket::NetworkError
Errmgs: "Unable to send a message"