What kind of EL key do you have? When you power on the machine with the el key does it boot into the game setup or does it just give you the error?
There is a few possibilities that these issues come up.
1. Wrong or bad el key ( I think they are only rated for 10 years lifespan)
2. Bad MPU board. Those boards are notorious for throwing errors at random. Bill Validator issues, printer not detected, top tower light stays on or won’t shut off, video won’t display, door or cash box door open errors.
They overheat and start baking those MPU boards in those metal ovens.
3 could also be a wiring issue with the jackpot reset key.
Have you just tried using the jackpot reset key. That should clear it… but the EL key should definitely clear it.
Try swapping boards from machines and see what errors you get.