Can't login after reboot

I installed 16.04 for my parents, I created an account during installation, then from that account I ran:

adduser useame
usermod -aG sudo useame

after which the new user was fine, BUT the original user can't login anymore (the password isn't accepted).

I launched a recovery mode and reset the password with:

mount -rw -o remount /
passwd useename

after which I could login, BUT after a reboot the sotuation as reverted back to not accepting the password.

I read about a few possible commands that could solve similar problems, but I'd like to understand what's going on, or at least how to diagnose this problem.