My print job has not printed and the middle LED (below the number buttons) is flashing?

The MFP paper trays are filled with A4 and A3 paper as standard. If the original document has a different format (e.g. A5 or legal), the device does not know which paper tray to use for the job. A4 is used automatically for the letter format.
The message on the status line that reads "Mehr Papier/More paper" (on older devices) is misleading and actually means "Which paper?"

Solution: Press the "eye" button (on the left of the display) or the "Statusmonitor / Abbruch" display button (depending on the model). After that, a picture of the MFD appears and you can select the paper tray to be used.