There are a few things this can be, after checking the lock as you have done the first is to check the small board that plugs into the formatter, the copy control board.
Remove that and on the reverse side is a memory Dimm, reseat that - if this fails or is dislodged then it can cause this board to stop communicating with the scanner unit.
Also the light patterns on the copy board and the scanner end can help, you should see a heatbeat, regular pulse at each end.
One other thing that can be tried is to manually lock and unlock the carriage, Configure Device, Resets, Lock Carriage.
Manually slide the lock - then power off the unit.
Power on , then manually unlock at the message.
Once unlocked, power off again, and then power back on.
In some cases this can clear the error if there are no issues with the board communications.
After this you would be looking deeper at light patterns, checking if scanner lamps are coming on, copy board power supply and other checks.