Sudo command failng with "conversation failed, auth could not identify password for [user]"

Issue

• sudo prompts for password repeatedly
• correct password is always entered
• root login and password works fine

secure:Nov  4 09:58:18   sudo: pam_unix(sudo:auth): authentication failure; logname=testuser uid=1266 euid=0 tty=/dev/pts/5 ruser=testuser rhost=  user=testuser
secure:Nov  4 09:58:27   sudo: pam_unix(sudo:auth): conversation failed
secure:Nov  4 09:58:27   sudo: pam_unix(sudo:auth): auth could not identify password for [testuser]