It's was clean install with the 1.18 version (old downloaded files), and then upgraded to 1.18.1 (including plugins). There were no users.
According to the logs, it appears as if the plugin tries to create the user on each login, but as it was created on first login attempt, this query fails.
Deleting the user "fixes" the problem, as the user can login again (and be created in the database), but any additional logins fail with the sql error of "duplicate key"