QVideoWidget and QMediaPlayer are showing Video on Windows, but not on Raspberry Pi

I want to play a Video on a Raspberry Pi 4 running Raspberry Pi OS in a Qt5 Application. I'm currently using QMediaPlayer and QVideoWidget as an output. It works perfectly on Windows, but when I build it for the Pi and run it, it shows a blanc Window.

Does it have something to do with Codecs not installed?

I use an absolute path in Windows ("D:/Projekte/Raspi_Video_Test/small.mp4"). I tried multiple path styles on Linux (for example "/home/pi/small.mp4" or "./small.mp4").

Sadly, QMediaPlayer doesn't show any Error massage. My code in mainwindow.cpp looks like this:

QMediaPlayer *player = new QMediaPlayer;
player->setMedia(QUrl::fromLocalFile(path));

qDebug() << "URL = " << player->media().canonicalUrl();

QVideoWidget *videoWidget = new QVideoWidget;
setCentralWidget(videoWidget);
player->setVideoOutput(videoWidget);
player->play();

This code causes following Debug messages on Windows:

URL =  QUrl("file:///D:/Projekte/Raspi_Video_Test/small.mp4")
Pause() {
Pause() }
Run() {
Run() }
Pause() {
Pause() }
Stop() {
Stop() }
Pause() {
Pause() }

and on the Raspberry Pi:

QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
URL =  QUrl("file:///home/pi/small.mp4")