Just a guess on my part, and only because something similar happened to me - not because I have any special idea about that type of leak.
Try the edges of the film back/door. On my 35mm, the foam light-seal at the hinged end of the door had degraded, and light leaked in on the take-up spool. It was probably like that for years, but I only noticed when I took some pictures without the half-case on the camera.
Try some tape along the edge of the door - my first test would be on the side with the take-up reel. If that helps, you should probably reseal the entire back. If not, the problem is likely elsewhere.