We found the root cause for the first user with the issue. We are researching the other users that still have issues.
The root cause of the first user, was that the user signed up in 2022 but never completed the sign up steps. This left an open record on the Mendix side.
The Mendix support team, removed that open record and the user was able to sign up successfully.
I’d like to mark this question as resolved. We identified the root cause and the users who had the issue were able to sign-up after Mendix removed their “old” registration record.