'The account in question is not registered' while subscribed??

[Sakuraluna]

Only thing I can think of is, do you not have a one-time password token by chance? I have one, so never witnessed this issue personally, but I seem to remember players without them experiencing temporary suspensions after logging into their account from unexpected IP's. If I'm right it's just until you contact them and is a precaution against getting hacked, or something... I don't know for sure. Software token is free though, last I checked.

Like someone else said, you could try logging into your account at mogstation and that should tell you the status of the account.