So don't always question first whether the new scanner is already defective. Most of the time, the fault lies in the handling, and replacing it with another new scanner won't help. The way you described it, it could be the shadow if the sheet is not completely pressed onto the glass plate. - You can try loading the A4 paper crosswise and cutting off the part protruding from the glass plate precisely, then weigh the sheet down completely with cardboard or whatever - then this edge should no longer occur within the cut sheet. Cheer up, you'll get it right. - The hinged lid of this scanner is handy when you want to scan something thick, but for single sheets (the main application of this scanner) it is rather a nuisance, see your problem.

There is no black border on my scanner. Something is wrong if you get a black border.