As an alternative to port 8080, you can specify the port that is being intercepted for the explicit HTTP proxy service.
For example, if port 80 is being intercepted and has the explicit attribute enabled, you can specify http://Appliance_IP_Address/accelerated_base_pac.pac
NOTE: It may be required that you select notepad as the application type to view the default PAC file. The proxy PAC file (default PAC file) is not configurable. The Accelerated PAC file is configurable and is blank. The proxy has port 8081 configured, but generally, it is not enabled. In SGOS 5.x, go into the Management Console > Configuration tab > Services > Management Services. Look for the HTTP Console server. See if there is a check next to the service in the Enabled column. If not, highlight it, click on the Edit button, and put a check into the "Enabled" button. Save your changes and click on Apply. Then you can go back to the URL and try again.