Dual stack any-address binding by QHostAddress::Any is broken here. Only IPv6 socket will bound. Force to IPv4 which is also used. Looks like QHostAddress::Any is broken (https://doc.qt.io/qt-5/qhostaddress.html#SpecialAddress-enum) This should be investigated in x11/qt5/qtbase or deeper. Spotted by ian@ and analysed with him. Thanks!