In 2021, HOI4 had several major updates (No Step Back update dropped late 2021). A common cause of crashes is a version mismatch.
: Since the mod often lags behind base game updates, players were frequently advised to roll back their HOI4 version via Steam. In 2021, this often meant rolling back to version 1.10.5 (Collie) or earlier to maintain compatibility with that year's mod build. hoi4 millennium dawn crash fix 2021