I think I now have a clue about this mysterious event. As mentioned,
the images were simply falling apart. Earlier today I remember noticing
that MSG-manager had disappeared from the screen. I had not actually
switched it off - you cannot do this by accident. After checking, I
started it up again and left it. Later I noticed the holes. Perhaps
somehow there was a second copy running unseen and therefore deleting
I reset the processing computer a short time ago, after leaving the
receive computer to accumulate files, and all now appears normal.