FYI: the commonly used term in English is "fogging", or "fog".
The bands of unevenness I would attribute primarily to fogging; the quality of the image of the negative strip doesn't really allow for much more to be said at this point, IMO.
I don't see the supposed fixer problem - only a specular highlight/reflection on the film surface in one frame, but better pictures might give more clues.
For now, the only thing I can see sure signs of is a light leak. It's not clear to me where it might have originated, but if I were to hazard a guess, I'd say in the 35mm cassette and probably not the camera. In-camera light leaks tend to have at least one straight edge to them somewhere.