This is almost always a hardware problem where the photo is not being transferred properly to Lightroom. The problem could be any piece of hardware in the transfer chain, including camera card, card reader or USB cable, hard disk and even memory problems. You need to try transferring the photo by replacing each with different hardware and/or trying it on a different computer to determine what the problem is.
thanks for the reply. My biggest concern is that the symptom may be spreading. I'm too worried even to start LR too often and to do things which may.harm my collection.
Is there any way of making shure that the pictures arrived safely on the disc and won't corrupt spontaneously?
If the process of importing the pictures is dangerous, I can cope with. But they shouldn't start behaving badly afterwards.
This has nothing to do with Lightroom. Starting it a bazillion times will not affect this problem or make it worse.
If your hard disk is having problems, or any other piece of hardware is having problems, then of course the problem could indeed be getting worse. Lightroom only reads and displays the information sent to it by the hard disk, and if Lightroom displays a corrupted image, then that is because the disk sent Lightroom a corrupted image. You need to diagnose where the problem is, like I explained earlier.