Looks like a flare problem to me. There's a tiny bit of sky discoloration in the leaves of the tree on the left side too, which reminds me of flare. The area outside the frame is dark (which is to be expected with slide film), which seems to indicate to me that it's not a light leak or something like that. That it was only in one frame, which was shot towards the sun, seems to indicate this is a flare problem, not a lab, camera, or other problem.
A lens hood might've helped this.