Comment 5 for bug 1766325

I have the same issue.
Both after adding a new user or doing a password change to a current user I have the same problem.

I cannot do a clean install because my server service provider had an older distribution for installation only, not 18.04

Luckily I have multiple user accounts with sudo access from 17.10, however this is a really annoying bug and a security problem for myself personally, as I am afraid that if I change my passwords something bad might happen.

Does Ubuntu do bounty for bugs? I would definitely pay/contribute to have this fixed?

I will continue looking for now.

I have changed my password multiple times and made it as short as possible to make sure the character input was correct.

I am willing to ugrade again if that will fix things.

Does this have something to do with my /etc/shadow and possibly the hash salt changing?

Error log:

Sep 7 15:37:31 localhost sudo: pam_unix(sudo:auth): authentication failure; logname=user1 uid=1003 euid=0 tty=/dev/pts/1 ruser=user1 rhost= user=user1
Sep 7 15:40:41 localhost sudo: pam_unix(sudo:auth): auth could not identify password for [user1]
Sep 7 15:40:41 localhost sudo: user1 : 1 incorrect password attempt ; TTY=pts/1 ; PWD=/home/user1 ; USER=root ; COMMAND=/usr/bin/test

sudoers:

#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults env_reset
Defaults mail_badpass
Defaults secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:

#includedir /etc/sudoers.d