The Wii U's MiiVerse service is currently offline. Trying to access it spits out an error message like the one you see above.
It's too bad because Nintendo's ambitious social network for its new console is pretty cool. I've been messing around with it for much of the day. It lets Wii U users post typed or hand-written messages (and drawings) to message boards dedicated to various Wii U games and apps. It lets people reply to each other, upvote comments, follow each other and even post screenshots and spoiler-tagged spoilers.
Sadly, it's not accessible right now.