This is a light leak, not flare. Possible sources: lens change while shutter not completely closed (you have an M2: shutter travel _is_ horizontal), shutter brake failing or separation between the leading metal edge of the shutter and the cloth curtain (can happen with time). I would shine a flashlight through the front of the camera with the lens off and view from the back with the door up. Move your eye around with the camera between you and the light and see if anything is getting through. If this were the first picture on a roll, I might also suspect the felt light trap on the film canister.
I had to have Sherry Krauter replace the shutter curtains on my M2 after the cloth shutter started to fail. Then had to send it back when the metal leading edge of the shutter pulled away from the curtain. These are 60 year old cameras now and this sort of failure is to be expected if the shutter has never been replaced (or even if it was replaced 30 years ago).
Ben