Generic error of GStreamer

[Marco Trapanese]

Hi,

Debian 6, 32-bit. From command line I issue:

gst-launch-0.10 filesrc location=/home/marco/Video/video.mp4 ! 
decodebin2 ! ffmpegcolospace ! xvimagesink

and I can watch the video.
Now I run the same pipeline in Qt4:

QGst::ElementPtr src = QGst:ElementFactory::make("filescr");
src->setProperty("location", "/home/marco/Video/video.mp4");
QGst::ElementPtr decoder = QGst:ElementFactory::make("decodebin2");
QGst::ElementPtr colorspace = QGst:ElementFactory::make("ffmpegcolorspace");
QGst::ElementPtr sink = QGst:ElementFactory::make("xvimagesink");

QGst::PipelinePtr pipeline = QGst::Pipeline::create();
pipeline->add(src);
pipeline->add(decoder);
pipeline->add(colorspace);
pipeline->add(sink);
src->link(decoder);
decoder->link(colorspace);
colorspace->link(sink);

QGst::BusPtr bus = pipeline->bus();
bus->addSignalWatch();
QGLib::connect(bus, "message", this, &Player::onBusMessage);
pipeline->setState(QGst::StatePlaying);

The error is: " GStreamer encountered a general stream error".

It seems the same pipeline. Why it should not work?
Thanks