This is a simple tray mis configuration issue. You need to make sure that the sending application attaches the correct document parameters to the job before it hits the printer. Once the printer recieves the job it will review the size and type parameters of teh document along with the tray configurations it has available. At that point it will decide to print to one tray or another or ask the user to perform an action so it can continue.
Try printing a sample job like the one you are working on from a different application. If the same problem happens then there is a tray mismatch on the printer. If the problem does not re-occur from a different application then there is a configuration issue on the original application that needs to be found and corrected.