There isnt a fix iirc, its been happening for a while

Firstly I suggest you connecting your wiiu via-ethernet, this will reduce connection errors

Then, here's some extra tips but unfortunately there's not an ultimate soltion for everyone: 1) Make sure Aroma and Inkay are up-to date 2) Make sure u have installed nimble 3) Switch to Nintendo Network and then switch back to Pretendo

4) Delete splatoon shared data and then reset wara wara plaza. Pretendo devs suggest doing this multiples times everyday until the error disappears Hope u'll manage to solve your problem!