Qml Module not found CPP Class registration with new QML_ELEMENT r

I try to register my custom CPP Class in QML with Qt5.15.0 and the new macro QML_ELEMENT, but the Module is not found. The Qt Creater help files describes the steps for the QML_ELEMENT. I checked the Qt Manual too, but without happy end (https://doc.qt.io/qt-5/qtqml-cppintegration-definetypes.html).

//testclass.h

#ifndef TESTCLASS_H
#define TESTCLASS_H

#include <QObject>
#include <QtQml>

class testclass : public QObject
{
    Q_OBJECT
    QML_ELEMENT

public:
    explicit testclass(QObject *parent = nullptr);

signals:

};

#endif // TESTCLASS_H


// testclass.cpp
#include "testclass.h"

testclass::testclass(QObject *parent) : QObject(parent)
{

}

The pro file looks like this:

QT += quick

CONFIG += c++11

CONFIG += qmltypes
QML_IMPORT_NAME = com.mycompany.test
QML_IMPORT_MAJOR_VERSION = 1

# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Refer to the documentation for the
# deprecated API to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
        main.cpp \
        testclass.cpp

RESOURCES += qml.qrc

# Additional import path used to resolve QML modules in Qt Creator's code model
QML_IMPORT_PATH =

# Additional import path used to resolve QML modules just for Qt Quick Designer
QML_DESIGNER_IMPORT_PATH =

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

HEADERS += \
    testclass.h

The Metafile gets generated with this content:

[
    {
        "classes": [
            {
                "classInfos": [
                    {
                        "name": "QML.Element",
                        "value": "auto"
                    }
                ],
                "className": "testclass",
                "object": true,
                "qualifiedClassName": "testclass",
                "superClasses": [
                    {
                        "access": "public",
                        "name": "QObject"
                    }
                ]
            }
        ],
        "inputFile": "testclass.h",
        "outputRevision": 67
    }
]

And afterwards I try to use this class in my Main.qml, but Qt Creator shows QML Module not found.

// main.qml
import QtQuick 2.15
import QtQuick.Window 2.15
import com.mycompany.test 1.0 // QML Module not found

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")


}

//Main.cpp
#include "testclass.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

Did I missed something or is it buggy?

** Edit **

I refactored the Classname from testclass to Testclass, but still I get the same error. Of Course I made Clean and rebuild. I'm using Windows 10, MinGw-64Bit and qmake.

//testclass.h
#ifndef TESTCLASS_H
#define TESTCLASS_H

#include <QObject>
#include <QtQml>

class Testclass : public QObject
{
    Q_OBJECT
    QML_ELEMENT

public:
    explicit Testclass(QObject *parent = nullptr);

signals:

};

#endif // TESTCLASS_H

//testclass.cpp

#include "testclass.h"

Testclass::Testclass(QObject *parent) : QObject(parent)
{

}

// Metafile
[
    {
        "classes": [
            {
                "classInfos": [
                    {
                        "name": "QML.Element",
                        "value": "auto"
                    }
                ],
                "className": "Testclass",
                "object": true,
                "qualifiedClassName": "Testclass",
                "superClasses": [
                    {
                        "access": "public",
                        "name": "QObject"
                    }
                ]
            }
        ],
        "inputFile": "testclass.h",
        "outputRevision": 67
    }
]

//main.cpp
#include "testclass.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

//main.qml
import QtQuick 2.15
import QtQuick.Window 2.15
import com.mycompany.test 1.0

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")


    

}