I tried every game in my Wii U library. Between several first party and third party games, all of them worked except Mario Kart 8. The disc is in perfect condition. I even ran my Wii U Cleaning Kit 3 or 4 times for good measure. The console tries to run the disc 3 or 4 times before giving up. Then it tells me that it's an "Invald Disc", rather than it being unable to read it for X amount of other reasons.
The DLC comes out soon and I purchased it the day it was made available. I was really looking forward to it. Does anyone know what to do?
Tomorrow I'll be running a test with a friend's Wii U. I will try his Mario Kart 8 disc in my console, and he will try my Mario Kart 8 disc in his Wii U to see which is the culprit.