Black rectangle on desktop (macOS Catalina 10.15.5)

Hello, I have the following problem, and any help would be appreciated. After I upgraded my Macbook Air to Catalina 10.15.5 a black rectangle appeared on the desktop screen, it covers whatever is under it, but only in the desktop. When I open apps, folders, etc., the rectangle does not affect them, however, it doesn't go away, and visible over whatever image I use for the desktop; it also covers icons of the folders and files in the desktop. I did a little research and it seems that the rectangle in my screen is different from other black rectangles that some people find in their Macs. I've tried to fix it, but nothing works. It's really annoying and I'm worried that it might cause more problems.

This is the black rectangle at the upper left corner:

It only stays on the desktop elements: