I had the same issue a month ago. Ended up calling the Gold Line the night before and the nice young lady told me that for some reason the system thinks that a US Citizen coming to the US needs to have an address in the US (we don't) and it messes things up. That was in spite of the fact that it was already in there from my profile and filling out the APIS data in MMB. She ended up forcing it in from her side and the system finally let me check in.