Dell E525w Error 005-121

When I put documents in the feeder to scan/copy/fax, it takes the first page, and then the second, but on the second page, it doesn't stop to factor the first page, as it is customary. 

Rather, it keeps going with te second page and towards the end of the feeding of that second page, stops and tells error 005-121, open ADF cover and clear originals. 

what could it be?