This is almost always a hardware problem. It can be any piece of hardware in the chain from camera card to card reader/cable/USB port to your hard disk or memory. You need to try transferring the photos using different hardware, one at a time, to eliminate each piece of hardware from being the culprit.
Thank you, dj_paige, for your quick response!
So you're saying that not the camera, not the lenses and not the computer are the problem,
but maybe the SD-card or the SD-card reader.
Normally I always use the built-in SD-card reader in my laptop. I'm going to try another way of importing, via USB-cable to the computer directly. Thanks!
I didn't say "not the camera, not the lenses, and not the computer"
I did say it could be the hard drive; I did say it could be the memory (those are parts of the computer), and if the camera is not writing the photos properly to the camera card, it could be the camera (which is rare, but you can't rule that out at this time).
But yes, its not the lenses