Remove the imaging kit, and then remove the imaging unit.
Warning—Potential Damage: Do not expose the imaging kit and imaging unit to direct light for more than 10 minutes. Extended exposure to light may cause print quality problems.
Warning—Potential Damage: Do not touch the photoconductor drum under the imaging kit. Doing so may affect the quality of future print jobs.
Insert the imaging unit, and then insert the imaging kit.
Print the document.
Do horizontal white lines appear on prints?
Contact customer support.
The problem is solved.