Horizontal line on printed page
When you use the flatbed the copy is perfect because the ccd sensor is under the glass cover and obviously is free form dust/ dirt.
On the other hand if you use the feeder the ccd sensor is located in between the roller assembly (not sure exactly for this model).
This means that the page is scanned by the ccd in the feeder, any dirt on the ccd will appear as a black line. I assume that you are feeding papers from the side and not from the top that is why you get a horizontal line.
A quick test to see if the ccd sensor is dirty is to feed the paper from the top side to the feeder. If you get a vertical line it is a proof that your ccd sensor needs to be cleaned.
The worst case is that the sensor in the feeder assembly has a tiny spot that is damaged and it is causing the black line.
So , first try the test, then open the feeder assembly, locate the ccd and clean it. Hope that this helps.