Thanks to all the suggestions. The same esta didn't have a problem last year and it's not causing problems to the other pax even if the hotel/stay information is now out of date. The BA agent tried himself a few times before giving up; he said that the esta is in order (but can they actually see it?), said the booking is all fine, but to go to the airport earlier. One thing I noticed before trying to check-in was that the pax in question did not have her BA account linked to booking; so I proceeded to update her profile including the passport number before trying to check-in. Maybe updating the profile with an upcoming flight causes this? Who knows.