It would be very helpful to see an image where you can see the strips you refer to.
You will start with the simplest ...
1- Replace the tonner. Some poorly recycled tonners fail like the one you refer to.
2- Clean the surface of the scanner glass. Where do you put the sheet to scan. So you remove any stains or dirt that may be spoiling that fault.
3- Cleaning the optical units of the scanner and the printer (Laser / Scanner).
If possible, attach a print image ...