DIAGNOSIS The scanner glass is dirty, or more likely, the white reference strip—the long, thin plastic bar that runs along the top edge of the glass inside the lid—is dirty or damaged. The scanner uses this to calibrate.
FIX Clean the flatbed glass thoroughly with a glass cleaner and a microfiber cloth. Don't spray directly on the glass. Then, lift the scanner lid and look at the underside of it. You'll see a white plastic strip. Wipe this carefully with a dry cloth. If it's stained or scratched, that's your culprit. You can also run the scanner calibration utility. On the printer's control panel, go to Setup > Scan Settings > Calibrate Scan System. Follow the prompts.
IF THAT DIDN'T WORK The scanner carriage might be sticking. With the printer off, gently move the scan head (you'll see it under the glass) from side to side by hand. It should move smoothly. If it's binding, there's likely debris on the rail. Use compressed air to blow along the metal rod it rides on.
HEADS UP Never place heavy objects on the closed scanner lid. It can warp the lid and misalign the reference strip, causing permanent shadowing on scans.