Can't join online servers

Whenever I try to join online server/multiplayer server it just says "Failed to authenticate your connection". I think this is because of my name, due to it saying Crectrum_ on the web-site but notCRECTRUM in-game. And since users can only change their username every 30 days, I would have to wait all those days before it gets fixed. Which is something I don't want to. So please help me with this issue or just give me my old username back.