So, I was just trying to join an online world on MC Wii U, other people are on it. My WiFi is very fast and I get this:
Can anyone help?
I believe this is simply a case of servers not functioning properly. If you don’t mind me asking, are you on Inkay or SSSL and has this issue been ongoing?
It might be worth checking your NAT Type: [Guide] Detecting your NAT type on Wii U and 3DS consoles
I was literally about to say the same thing. 
It has been only for MC Wii U, I play MK8 alot and it’s fine. I use inkay.
As @Joshua8600PNID stated, you should check your NAT type. There may be an issue with specific modules.
The specific module that is causing the issue is the PIA or (P2P) module that is failing to authenticate a connection with either the right keys or response times. This is a backend issue and it would most likely be on the server-side.
I would recommend ol’ reliable and simply disable Inkay as a module, turn off the console and activate it again. Or you could try disabling some plugins and trying again.
If this continues, please say so. I like solving cases. 