QPluginLoader doesn't accept plugin name

[Gourmet]

I'm weird. Just made plugin as described in books. All looks fine. In main application I try load it but got unexpected problem:

MainWindow::MainWindow(QApplication* a, QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QDir dir( QApplication::applicationDirPath() );
    QString abspath = dir.absoluteFilePath( "EmptyPlugin.dll" ); // name correct in abspath
    QPluginLoader PluginLoader( abspath );
    abspath = PluginLoader.fileName(); // name empty!!! ?????????
    QObject * plugin = qobject_cast<QObject*>( PluginLoader.instance() );
    QString errstr;
    if( ! plugin )
        errstr = PluginLoader.errorString(); // contains "Unknown error"
}

MainWindow::MainWindow(QApplication* a, QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QDir dir( QApplication::applicationDirPath() );
    QString abspath = dir.absoluteFilePath( "EmptyPlugin.dll" ); // name correct in abspath
    QPluginLoader PluginLoader( abspath );
    abspath = PluginLoader.fileName(); // name empty!!! ?????????
    QObject * plugin = qobject_cast<QObject*>( PluginLoader.instance() );
    QString errstr;
    if( ! plugin )
        errstr = PluginLoader.errorString(); // contains "Unknown error"
}

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1st time abspath is correct, but 2nd time it... is empty

tried this:

MainWindow::MainWindow(QApplication* a, QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QDir dir( QApplication::applicationDirPath() );
    QString abspath = dir.absoluteFilePath( "EmptyPlugin.dll" ); // name correct
    QPluginLoader PluginLoader( "" );
    PluginLoader.setFileName( abspath );
    abspath = PluginLoader.fileName(); // name empty!!! ?????????
    QObject * plugin = qobject_cast<QObject*>( PluginLoader.instance() );
    QString errstr;
    if( ! plugin )
        errstr = PluginLoader.errorString(); // contains "Unknown error"
}

MainWindow::MainWindow(QApplication* a, QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QDir dir( QApplication::applicationDirPath() );
    QString abspath = dir.absoluteFilePath( "EmptyPlugin.dll" ); // name correct
    QPluginLoader PluginLoader( "" );
    PluginLoader.setFileName( abspath );
    abspath = PluginLoader.fileName(); // name empty!!! ?????????
    QObject * plugin = qobject_cast<QObject*>( PluginLoader.instance() );
    QString errstr;
    if( ! plugin )
        errstr = PluginLoader.errorString(); // contains "Unknown error"
}

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effect is the same, no plugin file name assigned

why this can happen? PLEASE HELP!