Done.
Somehow, after restoring data.sav, one of the saves didn't pull up. I managed to do the following:
1. Delete the main save file.
2. Create a new slot with the same nickname.
After that, all the “_SUB” and “backup” files are linked to it and can be accessed again.
The old data.sav is no longer needed. I don't know how this will affect the game, maybe I'll write about it in a couple of days.
Damn it. This is true horror event..