Why do I keep getting an error when trying to publish with FrontPage?

I am trying to publish an update to my site and I keep receiving this meesage from FrontPage and it will not let me publish:

The web server does not have any authentication methods enabled. It asked for user authentication but did not send a www-authenticate header.

Does anyone know what this means or how to fix it? I have never seen this message before and have published many many times previously.

Thanks.