The firewall between the Filtering Service machine and the Remote Filtering Server machine is properly configured, as described in Firewall configuration. Make sure the block page port (by default, 15871) has been opened on the internal firewall. This allows Filtering Service to send block pages to remote users.

Any client machines configured to send requests through a proxy server have been configured to communicate with the Remote Filtering Server machine directly. See When client machines use a proxy server.