Ok.
It might get better in the future !

If you wan to get ride of QGraphicsDropShadowEffect :

void drawShadow(QPainter &_painter, qint16 _margin, qreal _radius, QColor _start, QColor _end, qreal _startPosition, qreal _endPosition0, qreal _endPosition1, qreal _width, qreal _height) { _painter.setPen(Qt::NoPen); QLinearGradient gradient; gradient.setColorAt(_startPosition, _start); gradient.setColorAt(_endPosition0, _end); // Right QPointF right0(_width - _margin, _height / 2); QPointF right1(_width, _height / 2); gradient.setStart(right0); gradient.setFinalStop(right1); _painter.setBrush(QBrush(gradient)); _painter.drawRoundRect(QRectF(QPointF(_width - _margin*_radius, _margin), QPointF(_width, _height - _margin)), 0.0, 0.0); // Left QPointF left0(_margin, _height / 2); QPointF left1(0, _height / 2); gradient.setStart(left0); gradient.setFinalStop(left1); _painter.setBrush(QBrush(gradient)); _painter.drawRoundRect(QRectF(QPointF(_margin *_radius, _margin), QPointF(0, _height - _margin)), 0.0, 0.0); // Top QPointF top0(_width / 2, _margin); QPointF top1(_width / 2, 0); gradient.setStart(top0); gradient.setFinalStop(top1); _painter.setBrush(QBrush(gradient)); _painter.drawRoundRect(QRectF(QPointF(_width - _margin, 0), QPointF(_margin, _margin)), 0.0, 0.0); // Bottom QPointF bottom0(_width / 2, _height - _margin); QPointF bottom1(_width / 2, _height); gradient.setStart(bottom0); gradient.setFinalStop(bottom1); _painter.setBrush(QBrush(gradient)); _painter.drawRoundRect(QRectF(QPointF(_margin, _height - _margin), QPointF(_width - _margin, _height)), 0.0, 0.0); // BottomRight QPointF bottomright0(_width - _margin, _height - _margin); QPointF bottomright1(_width, _height); gradient.setStart(bottomright0); gradient.setFinalStop(bottomright1); gradient.setColorAt(_endPosition1, _end); _painter.setBrush(QBrush(gradient)); _painter.drawRoundRect(QRectF(bottomright0, bottomright1), 0.0, 0.0); // BottomLeft QPointF bottomleft0(_margin, _height - _margin); QPointF bottomleft1(0, _height); gradient.setStart(bottomleft0); gradient.setFinalStop(bottomleft1); gradient.setColorAt(_endPosition1, _end); _painter.setBrush(QBrush(gradient)); _painter.drawRoundRect(QRectF(bottomleft0, bottomleft1), 0.0, 0.0); // TopLeft QPointF topleft0(_margin, _margin); QPointF topleft1(0, 0); gradient.setStart(topleft0); gradient.setFinalStop(topleft1); gradient.setColorAt(_endPosition1, _end); _painter.setBrush(QBrush(gradient)); _painter.drawRoundRect(QRectF(topleft0, topleft1), 0.0, 0.0); // TopRight QPointF topright0(_width - _margin, _margin); QPointF topright1(_width, 0); gradient.setStart(topright0); gradient.setFinalStop(topright1); gradient.setColorAt(_endPosition1, _end); _painter.setBrush(QBrush(gradient)); _painter.drawRoundRect(QRectF(topright0, topright1), 0.0, 0.0); // Widget _painter.setBrush(QBrush("#FFFFFF")); _painter.setRenderHint(QPainter::Antialiasing); _painter.drawRoundRect(QRectF(QPointF(_margin, _margin), QPointF(_width - _margin, _height - _margin)), _radius, _radius); }

drawShadow(painter, 10, 2.0, QColor(120, 120, 120, 32), QColor(255, 255, 255, 0), 0.0, 1.0, 0.6, width(), height());
don't forget setAttribute(Qt::WA_TranslucentBackground);
Final Resualt : https://onedrive.live.com/?cid=8882d1e3bc0f61ab&id=8882D1E3BC0F61AB!7415&authkey=!AH66ZieCJDqvXhk

Thank you for your answere. I've already tried that but it wasn't the solution. I've figured out that the problem occures when turning on OpenGL rendering. I guess it has to do with my hibryd graphic system. My main card has some serious problems so I can't run Optirun right now and I guess my secondary GPU does not support the required features (only OpenGL 3.X).
I'm not shure about that but as I said: turning off the OpenGL rendering solved that problem for me. Maybe it helps others.

Have a nice day everyone