Is this because of light leaking？ Or overdevelopment?

[nolindan]

I vote for a light leak in the camera.

If it were a development problem there would be a dramatic increase in contrast and not much fogging of the shadow areas of the image.

The light leak isn't in the processing stage as the clear rebate areas of the negative show no fogging, indicating the fogging happened when the film was inserted into the holder.

If the problem happens in the same area on horizontal shots (i.e. it rotates with the film) then the problem is in the rotating part of the camera's back.

[Jim Andrada]

Have you tried film from a different box to rule out fogging? But it's so nice (?) and neat and consistent I think there must be an issue withe the film back

[Huub]

thank you all for replying, your answers are all inspirational
so i tried again today, with a very careful inserting movement on my Chamonix and one negative upside-down development like Huub said.
But very sadly, it comes the same, same place on negative again...
It makes me think if it comes from my camera, every time it comes from the same place on negatives no matter if I'm using different lenses or shift movement, very confusing to me.

When you developed one negative upside down and one the original way and both negatives are much lighter at the topside coming out of the development tank, you can be pretty sure it is not camera-related, but development related. Also the fact that there is still some information in the less developed area's and the line between the less and well developed parts is pretty straight, makes met think that some way or another there is not enough developer in the tank. That there is still some information visible in the less developed parts is due to a bit of developer clinging to the negatives while reversing the tank.

If you have access to a darkroom you should expose some paper and make a paper negative and develop it in a tray and check what happens.