Originally Posted by Moscowflyer

Just to update: I found out the reason that I could not check in online to NY. The US still requires a covid vaccination confirmation so showing the certificate and signing a form at check in solved the problem (it wasn't the presence of a bookable upgrade in a later flight on the same itinerary which caused the problem as was initially claimed). I think that requirement ends in a week or so, after which the check in problem should also go away.