First Admin account not showing on login page
What I did (and what seems to have worked so far) is the following:


change username and home folder of the "missing user"
create a new user with the old username/home folder
delete the old user
use the new user instead


I think I should also have reused the old UID because I got a lot of files belonging to an unnamed "501" user that I had to chown to my new user.