Okay, so I'm convinced there is something seriously wrong with wireless communications on my 3DS. Everything is fine as long as the wireless is off. I just tried going to a friend's town on ACNL (I have it updated on my normal SD card). It works fine. But after a while, any wireless communication will bug my 3DS, then soft lock if I try to interact with it.
Please,
@NWPlayer123, please tell me you have something else up your sleeve that I can try. lol It's going to be a loooong time before I can afford to replace my 3DS, if ever. :/
EDIT: Just to see what would happen, I restored a NAND backup I have from last year when my 3DS was on 11.2, went through the A9LH->B9S update process, and went online with Pokémon Y. After a while, I got a communication error and my 3DS soft locked, and Rosalina produces the same error as usual.
EDIT2: Tried the previous again, but using an 11.1 emuNAND backup. No changes.