Can you post a picture of your MPU board at the U54 location. I believe that is 1 of the 2 chips right below the battery. Possible old corrosion or if someone socketed that chip maybe u bumped it and it needs reseated

Actually just looked it up theat chip is to right of batteries but I read that if the diode in you battery holder came loose that it may be causing your error. Are you using a remote battery holder? Or are your batteries still on your board? Google search pinwiki systems 9-11 MPU boot issues