@Aurich, SvC is working great on all 3 slots here. And the feedback we got from other users was the same, so I'm afraid that it's something related to your setup. Maybe the 4 slot or the cart itself. Do you have the chance to test it in another mobo? Try using the rompack released with only original games and try erasing the backup memory from service menu. Maybe that helps.
I don't have another mobo, this is a Neo Candy 29 with a 4 slot, never had a need to buy another motherboard.
At this point I'm thinking it might be less of a headache to just get whatever the best supported one slot is and be done with it, I don't need 4 slots with the multi anyways. Is there a consensus on what's the most compatible?