So...
It seems like whenever I lauch a app/program/game, my console freezes immediatly or freezes when loading the app. After many attemps, I got lucky and lauched the Settings app, but found out my Wii U was already updated. I tried lauching games with and without internet, external storage device, different users, but nothing worked. I doubt the CPU overheated since I rarely use my Wii U, it isn't covered/next to anything and the fans are still working. I could always try to format my Wii U but that would be a bit extreme.
Since I have no idea wether the problem is temporary/permanent and caused by a software/hardware issue, I kinda need help.
I'll probably post another message tommorow if anything changed.