Does the partial, off-center ring in your image continue on the next (or previous) frame?
If so, do the pair make a perfect circle? Of what diameter?
Did you roll your own film?
I'm thinking a possible one-off light leak somewhere, through something having a fresnel lens pattern or a concentrically ribbed plastic cap (Developing machine? Tiny hole in film canister = interference patterns = DIY pinhole camera?).
A tiny, perfectly round hole on the camera back in a place not covered by the film pressure plate?
Does the camera has a film reminder window and what does it look like?
Brainstorming here... :-)