I was migrating a server and rather than add all the users one by one, just copied over /etc/passwd and /etc/group. I totally forgot to get /etc/shadow and when I tried to change a user’s password, I got the error:
passwd: Authentication token manipulation error
To quickly correct this, I was able to run:
/usr/sbin/pwconv
and the /etc/shadow file was created correctly, now I can change user passwords as usual.