I have the same error. If i run Service Center Site directly from de server it works, but if i acess Service Center from other machine i receive the 403 error.
I never had this error until today. Means my installations was good but now i dont now whats happen.
Other thing...when i try to connect server from my machine with Service Studio i receive "Server was unable to process request. ---> Internal network use only - Access Denied."
Help me please