The same thing happens to me. I've been trying to play with a friend, and since yesterday, I get a communication error and the match ends. After that, I have to reset my router to be able to connect the Wii U to the Internet again, otherwise the connection always fails.
It's really ******* me off. The day before, we played just fine. The rest of my electronics work online just fine, so I can't really blame my router.
Edit: Just now, I was able to reconnect the Wii U without resetting the router, but I figure that the problem will continue anyway.
Edit again: It's completely random. I finished a match, and the next one had a communication error 1 minute into the game.