Technical Tip: Unable to configure port 80 for incoming HTTP or 443 for HTTPS in explicit proxy

The error messages pop up as follows when configuring port 80 for HTTP or port 443 for HTTPS via WebUI or CLI Console in Proxy Settings -> Explicit Proxy:

Change the admin port to another port via the CLI console, and Port 80 is successfully configured for the HTTP port:

config system global
(global)# show full | grep port
    set admin-port 80
    set admin-sport 443
    set admin-ssh-port 22

(global)# set admin-port 8800
(global)# end

config web-proxy explicit-proxy
    edit web-proxy
(web-proxy) # set http-incoming-port 80
web-proxy) # end

Related document:

Create or edit an explicit proxy