Why is there a transparent rectangle behind my object?

And how do I get rid of it?

This rectangle didn’t exist before. I got a few renders of my work in progress in the beginning, and everything looked fine. Now I have a gray/transparent rectangle behind my objects, no matter how I try to change the background. I can only see it in camera view, and it renders in my final images as well. I currently have a big cube in which my objects are placed, and the rectangle seems to conform to the cube however I rotate it (to stay behind the camera). I’ve checked the normals of the cube (they’re correct) and deleted it and created a new one, but the rectangle has stayed.

The cube is at a forty-five degree angle. The rectangle stays perpendicular to the camera view and fills in the corner. This is the rendered view, with Cycles. Any insight on how it got there and how to get rid of it?