I got hit by the same issue - submitted a ticket for livechat support (EU) and was told to fill this form (https://support.eu.square-enix.com/s...a=2&h=729633fa) which the help agent said is needed for them to escalate it.
Received a email reply the next day (approx 24hrs later) saying it was resolved and i was able to successfully key the registration code in Mogstation.

Hope everyone else can get this issue resolved soon!