Hi @anna.fasching,
Whenever a user fails to log in via a custom database connection because of an invalid email or username, it throws the “Failed Login (invalid email/username)” error.
Since the user does not exist in your legacy database, it failed to connect to the server and returned the error description as the reason for the failed login.
If you prefer to set a different error message, you could do so in your Login script. Please refer to the Custom Database Login Script Templates documentation.
Thanks,
Rueben