@Chris-Kawa turns out, you were spot on! I had some difficulties to compile my "fix" in release configuration.
So I went and rewrote the class, with your comment in mind. I intoduced a new signal and changed the overrides accordingly.
void signalEvent(QMouseEvent::Type t, QPoint p);
now, I don't need the timer, it also works in debug as well as in release mode...
virtual void mousePressEvent(QMouseEvent *e) Q_DECL_OVERRIDE{
emit signalEvent(QMouseEvent::MouseButtonPress, e->pos());
QPushButton::mousePressEvent(e);
}
works like a charm. Thanks!