ISA 2006 configuration

I’m facing a problem with the configuration of the ISA 2006.
The users cannot access the internet trough the ISA server.

The Event ID is 12209
Event Message
The ISA Server requires authorization to fulfill the request. Access to the Web Proxy service is denied. The configuration for outgoing Web requests should be checked. Contact your ISA Server administrator.

I hope that someone can help me figuring out this problem.