Hm, lot's of possibilities for this. I use a Nikon Z7 dslr and currently don't have problems. Things you've probably already checked:

1) The black frame is weird. Sounds like there's a mismatch between what the camera is set to capture and Kstars. Perhaps KStars is set to record an image that is larger than what the camera captures in its current mode?
2) If you do a straight capture using the camera using fits rather than native, does it return an expected image with stars and the correct dimensions?
3) I have never tried using the camera's native format for storage/transfer. Seems that there is definitely something different going on as you have experienced. I would tend to trust the fits format as being the "normal" way.
3) High enough ISO and long enough exposure to get stars using the dslr? I had an issue for a while when focusing where as the solver approached perfect focus it couldn't find any stars and would give up.

Cheers
Jerry

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