I had this issue when the printer was relatively new.
The jumper on the rear panel completes a circuit to tell the printer that there is a panel in place, instead of a duplexing unit. If I recall correctly, the jumper on the panel bridges two contacts, whereas a duplexer would make use of four contacts.
The rear panel does not fit securely enough when it clips into place for the two contacts to touch at all times. The design was bad, and I wrote an angry letter to HP about it. They should have used sprung tabs to maintain constant tension against the contacts.
***My solution was to apply dielectric grease with a toothpick, being careful only to apply a small amount on each contact, so that the circuit didn’t break with vibration.***