Could it be parental control settings?
I have none set, checked multiple times. :<
Some more info, the Japanese 3DS I use for Gateway I bought brand new, it's the Fire Emblem exclusive one. I don't remember what firmware it came with, but I coincidentally had it updated to 4.5 and then left it alone for a year because I use another JP 3DS as my main. Came back to use Gateway with it and it's never been able to successfully update the official firmware in GW mode. I've used Gateway before 2.1 too, and I tried to update the official firmware back then too, but when I did... the update would go through but it stopped reading my Gateway card so I couldn't play game images even though I was in GW mode. So I would have to revert back to the 4.5 Gateway and leave it that way in order to play games. Thought that 2.1 Omega would let me update the official firmware this time, but nope, it comes up as an error every time it shows like 5% on the download progress bar @.@;