Explicit Proxy + LDAP: requires a double authentication

Hi everybody,

I have a FG100E (5.6) working as explicit proxy connect via LDAP to a MS Domain Controller.

Everything works fine except that users need to authenticate on the browser 2 times  before they can see the content. After the first authentication the browser asks immediately to re-authenticate.

I'm not sure why this happens and I'm not sure how I can debug this.

Anybody can help me?

Thanks