Would be great if you could find that out, one of my testers had the same issue but I don't have that hardware to test with so have no idea where to start. I'll put a note for now that people with the Final Cartridge 3 need to disable it.
Okay, i have the answer now, how to fix this FC3-incompatibility. He wrote, it should actually be quite simple. The saver routine in the FC3 modul causes this error here in the game. To fix it, you have to set the save-address in the extended zeropage to default.
LDA#<$F5ED
STA$0332
LDA#>$F5ED
STA$0333
The code can be anywhere in the game, but it must be before JSR$FFD8.
That's all he wrote. To check, whether the fix works, you don't need a real FC3. You can also take an emulator and insert an image of the FC3 there, the behaviour will be the same. That's a good way to check, even if you don't own a real fc3 yourself.