It may mean several things technically but in our machine it is a slightly out of aligned / worn door interlock. We always clear the error by opening and closing the door or by pushing the door a little. The machine will return to normal instantly. Also try holding the START button in for 5 seconds.......this will reset the machine.
Ours started doing it after a guest broke the door handle off 5 years ago...........it has never got any worse so didn't investigate further as it was a minor inconvenience.
DaveB
December 2009