Tried to check in via browser and app. Kept getting error, it wouldn't accept my USA mobile phone number. Only after I ignored the instructions not to input country code did it work. So the solution is enter the country code without the "+" even if it says not to.
Also tried multiple times to check in another traveler using Firefox and Safari. After entering passport details and going to online check in I kept getting prompted again for the passport details. Finally got it to work using the app.