I cannot find much information on this error. Can you give us an example of your concerns?
I found this in a relevant support case:
After my investigation, I found that the “username is invalid. Email address cannot be used as username” error is being thrown when the custom database has the Requires Username and Import Users to Auth0 features enabled. Because of this, the provided username is rejected on authentication.
To workaround this issue, I recommend that you do one of the following:
- Modify your login script to remove the “@domain” part from the email address to fabricate an acceptable username.
- Alternatively, you can disable the Requires Username feature in your custom database, provided that this is not a strict requirement.