Photos not importing

Suddenly in the last week, my Photos application won't accept new photos via import on my MacBook Pro. The procedure I have always followed is:

I download the images (all in JPEG format) from my camera (Nikon D750) to a folder on my desktop, cull the ones I don't want to keep by opening them in Preview, then drag the good ones to my Photos application (in the toolbar), which opens and gives me the import screen.

Now, instead of accepting the images, Photos gives me an error message when I tell it to import the new images, saying: "This item cannot be added to your Photo library because it may be an unrecognizable file format or the file may not contain valid data."

The first time this happened, I imported 7 images in a batch of 10 and got an error message on the others. I opened the 3 that it rejected in PhotoShop and renamed them (kept them in JPEG format), and did the import again. It worked fine.

But today, that renaming trick doesn't work. I also tried changing them to tiff format - no luck.

No changes at the camera end of things, just a sudden rejection at the Photos for Mac end of things.

I'm running Big Sur 11.2.1.

Any ideas?