Everything seemed to work well at first. I was able to authenticate a single user using an external database. However, when I attempt to log in as a different user from the external database I get an authentication error: Invalid username. If I remove the user that was added to the wp user database table I can authenticate with a different user, but then it will not allow me to authenticate the previously successful user. It seems to only work with one user in the wp user table at a time.
Can you please help?