Using upload component -> empty portlet

##################
#PROBLEM SOLVED#
##################
Another developer has changed the vaadin version on the server (Vaadin 6.6.2)
I have developed with the 6.4.4 version. Now we use the same version and the problem is obsolete.
##################

Hi,

I am developing a portlet with Vaadin (liferay with apache-tomcat-6.0.29). I have a problem that occurs only recently:

Everytime I am using the Upload component the whole portlet is empty.

public class VaadinuploadtestApplication extends Application implements Upload.SucceededListener,Upload.FailedListener,Upload.Receiver{
	private File file = null;
	private Label label = new Label("-");
	
	@Override
	public void init() {
		Window mainWindow = new Window("Vaadinuploadtest");
		
		final Upload upload = new Upload("Upload", this);
		
		// Listen for events regarding the success of upload.
        upload.addListener((Upload.SucceededListener) this);
        upload.addListener((Upload.FailedListener) this);
		
		mainWindow.addComponent(upload);
		mainWindow.addComponent(label);
		
		setMainWindow(mainWindow);
	}

	@Override
	public OutputStream receiveUpload(String filename, String MIMEType) {
		FileOutputStream fos = null; // Output stream to write to
        file = new File("/usr/local/guseuser/tmp-upb" + filename);
        try {
            // Open the file for writing.
            fos = new FileOutputStream(file);
        } catch (final java.io.FileNotFoundException e) {
            // Error while opening the file. Not reported here.
            e.printStackTrace();
            return null;
        }

        return fos; // Return the output stream to write to
	}

	@Override
	public void uploadFailed(FailedEvent event) {
		label.setValue("Upload failed");
	}

	@Override
	public void uploadSucceeded(SucceededEvent event) {
		label.setValue("Upload succeded");
	}

}

There are no errors in the catalina.out:

I tested the same portlet on a fresh liferay installation (same version) and it worked. But what could cause such a problem?

Best regards,

Johannes

The empty one:

Exactly the same .war-file: