I ran into the following error when trying to change my password:
passwd: Authentication token manipulation error passwd: password unchanged
I found the following pwck linux command which validates the /etc/passwd and /etc/shadow config files, and advises of any problems.
From the man page:
NAME pwck - verify integrity of password files SYNOPSIS pwck [-q] [-s] [passwd shadow] pwck [-r] [passwd shadow] DESCRIPTION pwck verifies the integrity of the system authentication information. All entries in the /etc/passwd and /etc/shadow are checked to see that the entry has the proper format and valid data in each field. The user is prompted to delete entries that are improperly formatted or which have other uncorrectable errors.
Checks are made to verify that each entry has: · the correct number of fields · a unique user name · a valid user and group identifier · a valid primary group · a valid home directory · a valid login shell The checks for correct number of fields and unique user name are fatal. If the entry has the wrong number of fields, the user will be prompted to delete the entire line. If the user does not answer affirmatively, all further checks are bypassed. An entry with a duplicated user name is prompted for deletion, but the remaining checks will still be made. All other errors are warning and the user is encouraged to run the usermod command to correct the error. The commands which operate on the /etc/passwd file are not able to alter corrupted or duplicated entries. pwck should be used in those circumstances to remove the offending entry.
Link to the askubunu thread where I found out about the pwck command: