Nikon D610 card error, camera or card the culprit?

I have a recently purchased D610, half a year old or so, and it's been shot a few thousand times, 3000-4000 shots approximately.

Just recently I got a "Card Err" in slot 1, with a 64Gb Lexar Premium 300x (a Lexar 300x 32Gb in slot 2). I tried taking the card out and putting it in again, turning the camera off and on, and it worked fine for a couple shots. Turning the camera on the next time I got the error again. I switched cards, card 1 to slot 2, card 2 to to slot 1, and the error now was on slot 2, the same card that had the error previously. I tried formatting the card, but that did nothing, the files were still on the card when looking at it on a computer.

I tried the same card in my Fuji X-M1, and it worked fine, pictures showed up and I could take shots normally. Shooting and deleting the shot worked normally. But putting the card back in the D610 it didn't work anymore, same error as before.

I was finally able to format the 64Gb card, but only after taking card 2 out of slot 2. Now it seems like it's working normally again. I use card 1 for RAW and card 2 for JPG + Videos, if that matters.

It would seem to me that this was a problem with the card, but I'm not 100% sure since it worked fine in the Fuji.

All I'd like to know is if the problem is with the card or the camera? The camera is still under warranty so if it's faulty I should probably show it to Nikon before the warranty runs out. Any similar experiences? Any help is appreciated!