It has become evident that the square aperture in front of the CCD chip can shift - or rather, that the CCD camera can shift relative to the mask. The mask is present to help the frame-transfer procedure by shading the part of the frame that holds the transferred image during readout. If the CCD was not half-shielded the 'smearing effect' would occur due to illumination during readout. The mask is a set of adjustable blades, apparently that are fixed wrt the telescope body. The CCD itself is held to the body by a thread and the correct position is indicated by two steel pins that should touch, visible from the outside. It has occurred that the CCD has 'unwound' and this caused 'dark corners' to appear in the images. The camera had indeed 'unwound' a bit and was rotated back into proper position.
Following that incident a 'black bar' has appeared in the side of the image - perhaps due to slippage of the camera wrt the blades, or the blades' motion.
The issue is further discussed here.