I think I have the answer for you (or am pretty sure), as I have had the same problem. I also use the 645n2, and I see from the first negative that you used the 45mm lens. This has the petal shaped lens hood, and I am sure that you have put it on slightly wonky. If you don't match up the white dot on the lens, and the white dot on the hood and rotate clockwise so as the word "Pentax" is on top, your hood will not be on straight. If you take a look at your lens hood you will see my thinking behind this, as the dark areas on your image mimic the dips either side of the petals on your lens hood. Could it be stray light entering the lens?, or the hood obscuring the image area? All I know is that by making sure that my hood is on straight I have not had the problem return.
I could be wrong in your case, but ask yourself when you retested the camera if you used the same lens, or correctly fitted your lens hood, if fitted at all. It is easily done.
I hope this helps.
All the best