Hi Paul,
in the picture I didn't exchange the real background by the rectangle, because the problem did also exist without the background image.
Clock and some elements in the header are not updated correctly, but elements in the footer are displayed correctly. The fragments on the clock are the remains of the hour handle.
No, they don't depend. The both problems do not occur at the same time. The error message is displayed any time I open or close a dialog in the bottom part. The background is changed independently from this, because it ist triggered by some external event, which is coming from serial port.