K2 Designer will not open from any machine apart from the server

Hi,

ANy ideas what this could be:

I can open K2 Designer from the Server itself, but when trying to open the K2 Designer URL from a Desktop anywhere else, it prompts for a Windows Security login. When you enter the login credentials it just keeps going back to the login box and won't let you in. After 3 tries i get:

Not Authorized

Not Authorized

HTTP Error 401. The requested resource requires user authentication.

I've got IIS set to Forms Authentication and Anon on the Designer site and Windows Auth only on the Runtime site.

Any reason why it won't let me in?

I have full permissions to the K2 Server, so that should not be an issue

Thanks