Unfortunately, but a repair might be unavoidable, however, do you have an External Hard Drive connected and a SD card inserted in your Wii U console, as either could be causing the freezing problem.
Another thing it could be if you have your console AC Adapter being powered by one of those multi-outlet power bars and also have other electronics all using the same power bar, as that could lower the electric power output just enough to keep the Wii U console from being able to fully load the system files.
But if removing the External Hard Drive, SD card and just plugging the Ac Adapter directly into a wall outlet and it still freezes, then unfortunately you might have to contact Nintendo again to set up a repair, sorry.