Every time the printer is switched off it forgets the setting for the paper size - even when it is still energized by the power supply unit. Therefore, I have permanently to change the paper size from "letter" to "A4" before copying is feasible.
I have already examined the main board. There is obviously no backup battery needing to be replaced. The printer has been manufactured roughly in the year 2000.
I got rid of this problem by resetting the printer to the original state.
Unplug printer which is turned on.
Hold the keys 6 and # (lower right key).
Plug in the power cord while still holding the keys.
Do not release the keys until some instructions appear on the screen.