There was a missing piece for me that may help some of you, that enabled me to get into early access after encountering this error.

I was following the instructions of the email of the registration code to go to the Additional Expansions button and register.

However, early access is from a separate email with a separate code. You are looking for the one that offers preorder bonuses, and a different button on Mogstation. The icon showed the bonus earring. The early access is bundled with the earring.

Enter the code there and you will be able to get into Dawntrail content.