Those that had used the returning Psychic-type Legendary Pokémon on any of the game’s core modes and then taken the fight online, had seen their Global Smash Power “incorrectly identified as unauthorised data.”
At which point, players were prevented from connecting online presumably as such detection would have suggested that they had tampered with the game in some way.
“After using Mewtwo in Smash Run (a mode exclusive to the Nintendo 3DS version), Classic, All-Star, Stadium, or Trophy Rush, and then connecting to the internet in the game, your Global Smash Power may be incorrectly identified as unauthorised data,” Nintendo previously explained within an in-game notice. “From this point on, it will not be possible to connect to the internet from within the game.
“We are currently preparing a software update to rectify this issue. Until this update is available, we recommend users refrain from using Mewtwo in Smash Run (a mode exclusive to the Nintendo 3DS version), Classic, All-Star, Stadium, or Trophy Rush.
“No issue has been identified with using Mewtwo in Smash or Smash Tour (a mode exclusive to the Wii U version), as well as in Online mode as long as nothing was previously identified as unauthorised data.
“We will send you more information when the software update is ready.”
Thankfully, it’s now all sorted so pulverise your foes with Mewtwo to your heart’s content!