Strange round borders on camera lens in render and viewport

Hey folks

I got a lens issue, getting these weird round edges ony my cam view, no idea where they come from. I tried to rule things out, but nothing made them disappear. They move when I change the camera focal length though, so guessing it’s gotta have something to do with the camera setup? I could make them disappear by moving the focal length in far enough, but I really don’t want to change this camera setup. Any ideas where this could be coming from? I went through all the lens/camera/postprocess settings I could think of, it keeps persisting no matter what.
Thanks.

PS: I posted this in Cinematics & Media before, but it got removed for wrong category, no idea how that isn’t a cinematics issue, but I hope it won’t be removed again in here. It’s an issue I’ve been trying to find answers all over the web and found none, and I gotta solve it for a client job, so please don’t remove it again, it’s utterly annoying enough already.