jam (230.11) see 230.11 jam
230.11 jam – The problem turned out to be that the "Duplex/manual feed sensor (S1)" flag would not rotate freely. This is a rotating plastic "flag" inside the tray which, when a piece of paper goes through that part, breaks a sensor. The fact that the manual said it was supposed to rotate freely but appeared stuck in place was my first clue. Peeking inside the tray mechanism I finally spotted a wadded up piece of paper.
I had to disassemble the tray (which involved unsnapping a part that was only held together with 4 plastic "dogs" that fit tightly into 4 corresponding holes and had to be pried out oh so gently to avoid busting any one of those rather delicate molded plastic dogs) to find and remove that wadded up remnant of an envelope which was jamming that flag's interior mechanism (and likely preventing the free flow of paper through that area as well).
Thanks to the Lexmark X544 Service manual, page 82.