Thank you for providing this, @jmusits ! - I can see you have been discussing your issue with our Support Center and that you were able to discover that switching to the Universal Login allowed you to log in to your app with the test user coming from your database.
Another topic that popped up from the discussion is the ability to log in after the user has been successfully migrated to the Auth0’s Username-Password-Authentication database. To allow this, both connections, the custom database connection (to further allow remaining users to be migrated), and the Username-Password-Authentication (to allow already migrated users to log in next time) must be enabled for the app.
On the New Universal Login screen, we can only provide users with one database connection at the time, thus, to authenticate the user through one of them, an /authorize request should contain the connection name parameter - Authenticating users through a specific Connection (database, social, etc.).