i have the same issues, i'm getting a lot of errors on other online things too

I fixed this by opening the miiverse applet then closing and going back to the game. you should also disable your wifi firewall and set up SSSL. (SSSL is meant for unmodded consoles but I couldn't get splatoon to work without it.)

ok so that worked, but i had to connect through my phones hotspot, probably because spectrum doesnt allow custom dns servers for some reason.