unexpected SSH input_userauth_request: invalid user

We have a user that suddenly cannot log in via SSH on a Linux box. The error output in /var/log/secure is:

Invalid user foo from xx.xx.xx.xxinput_userauth_request: invalid user foo

Some additional details:

• The user has a shell correctly specified in /etc/passwd

• The user has a home directory with the correct permissions

• We made no changes to sshd_config, verified by the mod date

• No changes to user's .bashrc or .bash_profile

• The user cannot log in via password or key

• I've restarted sshd, and restarted the box

• The user's shell history shows no commands that might have caused this problem

• Ditto for root (used by only one person)

• VERBOSE output for sshd gives no other details

• No other user has this problem

I've done an extensive search, and have not found where else to look to identify the problem -- any insights would be greatly appreciated, thanks in advance.