Unsolved CPP QML Singleton registered with qmlRegisterSingletonType fails to load

[Lolleko]

I'm trying to register a QObject as QML singleton via cpp.
The singleton is located in a shared library not in the main application.

My Object looks something like this:

// header
class SHARED_EXPORT Singleton : public QObject
{
    Q_OBJECT
public:
    explicit Singleton(QObject *parent = nullptr);

    void method1(const QString &message);

    static Singleton *globalInstance();

signals:
    // some signals

public slots:
    // some slots

};

// source
Q_GLOBAL_STATIC(Singleton, theInstance)

Singleton * Singleton::globalInstance()
{
    return theInstance;
}

The singleton provider is located in another header

namespace QMLSingleton
{
    static QObject *globalInstance(QQmlEngine *engine, QJSEngine *scriptEngine)
    {
        Q_UNUSED(engine)
        Q_UNUSED(scriptEngine)

        QObject *instance = Singleton::globalInstance();
        QQmlEngine::setObjectOwnership(instance, QQmlEngine::CppOwnership);
        return instance;
    }
}

I register the singleton in main via:

    qmlRegisterSingletonType<Singleton>("Package", 1, 0, "Singleton", QMLSingleton::globalInstance);

everything compiles fine and qmlRegisterSingletonType works without any errors.

I also have auto completion in the qml editor showing all signals / slots of the registered singleton.

But once I load the main.qml file of the app this errors shows up

QQmlApplicationEngine failed to load component
qrc:/main.qml:29 Non-existent attached object // in line 29 I am using the registered singleton 

I also tried less complex examples (like this one https://stackoverflow.com/a/26782682)
but even if I declare a really simple qobject and provider in the shared lib or directly in the main application and register it I get the same error.

Any ideas whats causing this issue?

EDIT:

I also tried to create the singleton without a singleton factory (Q_GLOBAL_STATIC)

namespace QMLSingleton
{
    static QObject *globalInstance(QQmlEngine *engine, QJSEngine *scriptEngine)
    {
        Q_UNUSED(engine)
        Q_UNUSED(scriptEngine)

        return new Singleton();
    }
}

... same result