It seems that something in Peach's moveset is causing trouble when players take her into online mode. A lot of Peach players are receiving a one-day ban for supposedly cheating, but there's no cheating going on. It seems that Peach's down+B move, if done right at the start of the match, is causing the game to think that something illegal is going on. Long story short, steer clear of Peach online play for now until the issue is dealt with.

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