I have a custom settings file which I write with QSettings as:

    auto format = QSettings::registerFormat ("iniformat",  readIniFile, writeIniFile);
    QSettings settings("Settings/local.info",format);

since version 5.4 the conversion in writeIniFile :

bool writeIniFile( QIODevice& device, const QSettings::SettingsMap& map )
{
    QFile*file = qobject_cast<QFile*>(&device);

fails to convert device into QFile*. The result is 0. This was working at least in 5.2.1, now it doesn't

I have attached a minimal test project. Set breakpoint at line 11 and you will see the problem - file is 0, as conversion failed.