Thanks Barry. I started at the beginning of the images, and removed MM from all corrupted clips, then re-applied MM, checking the clip playback in MM and again in the timeline, to ensure that it ran properly, then saved the project as replacing the existing one, and went through to the end, doing this at each defective MM clip, and somehow it worked. It took a bit of time, but fortunately, most clips with MM were already OK.
I also discovered that you cannot apply backlight correction with Magic Clean to a clip which already has had MM applied..it has to be in the reverse. This is not mentioned in the Manual, and perhaps should be, in the next edition. Doug