it might be an issue of, like, what security the hotspot is trying to use.

with the Win10 hotspot, on the 3DS i could connect to it just fine as expected when--for the "Security" option--i selected "WPA2-PSK(AES)".
but doing that with the linux hotspot that results in the aforementioned error.
however! instead selecting "WPA-PSK(AES)" seems to work, apparently.