I think this is new. From ANA website:
(International flights) Issues in which online check-in for itineraries departing from Vietnam cannot be used
Thank you for using ANA.
Currently, online check-in from the ANA website is temporarily unavailable for itineraries departing from Vietnam.
Regarding the reopening time, we will inform you again on this site.
Online check-in using the ANA app is still available.
We apologize for any inconvenience this may cause to our customers, and appreciate your understanding.