Samsung ML-2851 signals red light and do not print

Problem: Printer Samsung ML-2851 takes paper only from the upper tray

The cause for printer ignoring the main tray and using only upper tray was a paper that was the last one in the rack. That single paper was slightly thrusted and it should be removed away from the tray. Somehow it made printer think that the whole main tray was missing even though it was correctly put in it's place.