When I was to changed "root" password then come up message like this
"passwd (SYSTEM): Unexpected failure. Password database unchanged", try adduser, then his come up again this message "UX: useradd: ERROR: Cannot update system files - login cannot be created."
check your / space, house keeping if its full, then change your root password with
# passwd -r files root
if doesn't work, this solve maybe help.
..also check the users home dir./mount it may be 100%
@mrwolfer1: definitely yes, thanks for you feedback