Definitely looking to be a server issue. The message that shows up is being produced by the server itself - the entire in-game registration window is actually just a webpage on spore.com, https://community.spore.com/community/auth/registerNew (note that the page won't load unless it's accessed by the game) and that webpage contains the error message.
So it's definitely not some sort of connection issue or anything, and it does correctly identify when the email/password is incorrect or a Spore persona already exists on that account. The rest of the Spore server seems to be operating normally.
I'm gonna play around with it some more, but there's not a whole lot I can do as I don't work for EA or Maxis. Might need to try and get in contact with them, if there's no response on this forum then I'll see if can find anyone with contacts at Maxis. Last I heard the live ops team (which manages the Spore server) was mostly working on other Maxis products so not sure who all is still there, but there should be someone who can look into it.
I'm guessing there was some backend change, possibly on the Origin side, that broke the connection between the Spore server and the Origin server, thus it can't validate whether an account owns the game. It might be possible to manually upload your CD key into the Spore server, as this was possible years ago (performed automatically by the game), since the disc version did not require use of Origin. Going to research that a bit, in case we don't hear from Maxis soon.