Hey, Grin!
I'll take a wild guess here, did you have a NA account? Don't forget to set your Square Enix account user settings / profile to NA too, even in the preferred language setting-- actually, just set everything you could set into NA region, including the address bar of your browser, does it spells EU somewhere? Replace it with NA if it is.
Afterward, you should log out and go to the http://preorder.na.finalfantasyxiv.com/us where you should continue as usual, logging in and such.
Good luck!
nb: If it's continue to produce error messages, it could be that the server is congested, it happens the day before, but eventually you should get through and receives your 20 digits true code.
I went over every possible spot something might be set wrong in my profile or account. It's all English (US), and the address I'm linked to is preorder.na. I still get the Error 2: The code entered has already been used. AND the game is still no longer registered to my account.