Grey "dot" in viewer and exports

I have a strange grey rectangle in a viewer and even in the exported file. When I move or zoom the video, it stays in the same position and size. It looks like some kind of marker, but I couldn't find a solution.

IMG_7526.jpg (1008.27 KiB) Viewed 443 times

Any ideas?

In the Workspace dropdown menu click on Data Burn-in and make sure nothing has a tick against it.