• The connector returns a 404 Not Found in any failure scenario, whether:
  • The user does not exist in the external system, or
  • The user exists but provides invalid credentials.

This design is intentional and exists to prevent user enumeration attacks by not revealing which part of the login process failed.

Because of this security restriction:

• You cannot display different error messages on the hosted login page for different connector failure scenarios.
• There’s no way to pass additional custom data or error context from the generic connector to the hosted templates for display purposes.

You can read more details here:
Generic Connector - Response