Canon MF4100 have toner but display "no toner"

Problem: Although toner FX100 is in printer/fax/scanner Canon MF4100 - it displayes toner error "insert toner catridge, lift cover etc"

Solution: First remove all papers stacked for printing, then remove toner catridge and of course return toner back inside. One paper was in printer set to print - it should be removed, somehow it made canon to think that there's no toner.