Solution:

Open the front cover of the multi-folding unit.

Pull the multi-folding unit out.

Turn the N11 knob counterclockwise until the blade appears.

The blade is located in the right part of the multi-folding unit.

Wipe the tip and top of the blade with a soft dry cloth.

Be careful not to damage the blade.

After cleaning, restore the machine so that it resumes operation.

Apply multi-sheet folding and print 3-5 copies. The paper stain will disappear.