Render in cycles has visible squares

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2.81a is the official one currently.
I have seen other people with strange, un explainable, artifact in their render. Sometime it’s a problem with the video card.

Are you using experimental Blender features?
What kind of render device are you using CPU or GPU ?
In the Blender preferences there is a systems tab, where you can select Cycles Render Devices
Change them To none, OpenCl, or others.

See also this thread : No rendering