HP ProCurve 6400cl Series Access Security Manual page 146

RADIUS Authentication and Accounting
Switch Operating Rules for RADIUS
6-6
When this type of failure occurs, the switch prompts the client again
to enter a username and password. In this case, use the local user-
name (if any) and password configured on the switch itself.
Zero-length usernames or passwords are not allowed for RADIUS
■
authentication, even though allowed by some RADIUS servers.
■ TACACS+ is not supported for the web browser interface access.