This should solve most of your questions
https://gbatemp.net/threads/guide-community-noob-guide-to-wii-u-hacking.451297/
However, try to refrain from buying a used wii u as it may be 'bricked' where someone may have failed to install hacks and nearly unusable.
-Try to get the seller of the wii u...
I have the same error code as you
I used Splathax,Gearhax and Octohax then lost my wii U for about 2-3 Months ago and I find my Sploon with the same error code :(
I plan to call/email Ninty support soon and see what they say, wish me luck :P
@KapuDaKoopa
You need to update the AIO link to https://github.com/OatmealDome/Splat-AIO-2/releases
It goes to https://github.com/seresaa/Splat-AIO-2/releases which doesn't work
Only thing drive me crazy is all these new horror games nowadays is its so dark, all time. I can't even play them. I understand they trying to create spooky but be creative a little, not just darkness with pop out scare scenes.