Hi Steve,
I recommend you to first save this Registration(s) to the same Reg button(s) again, but without having ticked the "Song" checkbox. Then save the Reg BANK using a different filename.
Now test the Reg(s) of this new Reg Bank (when no MIDI Song is called up by the Regs).
Test also to load the regarding MIDI file MANUALLY into the Song Player.
This way you should be able to determine whether a Registration is faulty or the MIDI file linked in the Reg.
Best regards,
Chris