For those who want to know what exactly the message says:
"The report (the save file I guess) is broken.
For more information, please refer to the instruction manual".
I guess knowing how to read Japanese was not useful at all for solving the problem since you've figured it out already :yaysp:
I use an SDXC in my 3DS but I've formatted it to use only 31GB as a fat32 partition. the reason for that is, that fat32 supports at max. 32GB in one partition and the way it formats more than 32GB is so complicated that the 3DS and any other devices take ages to read or write data on the SD...
It freezes at random black screens, for example, if you open your notebook, then it shows, or it freezes before showing it. I would say, it needs a patch to enjoy playing it...