I am assuming the US address requirement is for covid-contact reasons, as they specifically say it is the address where you will be spending the first night after arrival in the US. In our case, we are flying to SFO which is not our home, so I have entered the local (San Francisco) address of our daughter where we will actually stay, rather than our home address in Seattle. This worked fine on the other booking (LHR to SFO only, no MXP involved) when I entered it through MMB there, before turning my attention to the newer booking.

So I cannot blame a system-wide glitch. The one difference, apart from the point of origin, is the fact that I used AS miles for this booking rather than Avios. I will try again in a few days, and if I doesn’t work then I will wait til January when things should be calmer, and try calling. But I suppose I need to use the general US line rather than the BAEC US number I usually call, as this booking is not in any way tied to my BAEC account.

I would just like to get it sorted so we can do OLCI, as the first flight (MXP) is an early departure (8 am) so any additional time needed for document check would mean showing up at the check-in desk around 5 am. And I would need to pay to choose our seats on both flights well ahead, as we do not want to rely on the random assignment we would get at the airport.