I had front-focus problem whith mine 645 super. It was just the mirror that needed to raise a little bit. Very easy to adjust,there is a tiny little hole in front low right.
The screw is locked by a lot of black glue or something. No problem, if the screwdriver fits perfekt.
In case of frontfocus and to raise the mirror, turn left. Backfocus will be opposite. Very simple mirrorstop in these cameras and easy to adjust, one plastic piece, a spring and one adjustmentscrew.