I was moving my three 3DS devices to Pretendo’s network and adding their friend codes to each other. I was doing this locally, and would have errors after registering.
I finally narrowed it down to the old version 3DS causing the problem, and further narrowed down that setting a favorite software was the root of the problem. It didn’t matter if the software was a legit title or homebrew. But by not setting a favorite software it stopped causing the errors.
The other two, an LL and New LL, can list the favorite software just fine.
Is the old3DS a JP console?
It might just be a thing with JP and other region consoles not able to work properly locally for friends if a favorite title is set.
But if they’re all the same region, then that’s just weird lol
I believe the two models and their firmware have different inbuilt connection modules. There may be some esoteric homebrew fix, though I am not sure. 