Background strange mirroring effect

Hi, I have a scene inside a spaceship. At one position I have a window and since today I have the problem, that the background (Skybox) has a strange mirroring effect.

I’m using Unity 2023.2.20f1 with a camera and Cinemashine Virtual Camera. If I check the same position from scene view everything is fine. If I’m in game view I see this doubling effect.

Everthing was fine for month and now I have this effect, but no idea what I could have changed to case this.

Any idea for the reason? I guess it’s somehow a camera setting problem.