Did that happen immediately? Haven't had any problems so far
It was fine for a few months, then the problems started - possibly linked with an update - I tried deleting the data and starting again, no luck, different copies of the game, installing it in different places, nada. I wouldn't worry, it seems to be a very niche problem as far as I can tell, possibly related to the fact my Wii U is quite old now, bought it second hand last May and they'd had it a while so it could be one of the very first few European ones.
Only thing I've found online was that it might be related to the flash chips they use in the system memory, people with similar problems have had them replaced and it has fixed it. I'm loathe to lose save games again though, already nearly won all cups three times now!
I just don't want to be without my Wii U as I occasionally review games on it and can't just abandon it for three weeks again. Mind you, I wonder if next time I ring Nintendo about the problem I drop the fact I review for a national newspaper on them, they might reconsider how quickly they turn it around. Got to be worth a go.