When users are being loaded at the same time that another thread is trying to authenticate a user, it is possible that authentication fails due to the update to the users database.
With the attached configuration, run the following command on one terminal.
At the same time, start the following on a different terminal.
This will cause the following message to be logged in the first terminal.
This was only reproducible on 2.1 as the authentication was made thread specific in 2.2.