Have you eliminated the possibility that it might be fogged film?

I can't tell exactly where the light is coming from but, in my experience, flare happens more often when you are shooting directly into a bright light but it doesn't seem like there is bright light coming from the scene.

I would try shooting the same film in another camera or shooting the same film and camera with a different lens. If those two tests are inconclusive, you could try wrapping the camera in aluminum foil and gaffer's tape to seal out the light. If you have an Ever-Ready case for the camera, try using the camera inside the case.

Even if my idea is wrong, it is a simple matter to eliminate it as a possible cause.

Always try to eliminate the simple things first.