No idea right now.
I did check on my own bank of saves and I saw that 0x483F got altered when the file was loaded, but the checksum calculation worked correctly. That'd be what is causing the 0C block to show up as bad (still have to figure out what is causing that change).
(edit: this won't have any bearing and was related to loading a save and showing the current box where something failed)
Try transplanting data from other backups to see what area of the save is the troublemaker.
Edit2: Was that 0x483F set to 0x1F? Might be the reason why (ie, last used the PC on a non1-31 box and exited, ie battle box?) I just committed a would-be fix so that the checksum error won't appear and the &0x1F is really using a signed byte instead (to indicate battle box). If that doesn't work, maybe keeping it below 0x1F might be the fix...