You closed port for outgoing traffic but allowed for incoming traffic?
I close outgoing port 8443.
My internal clients are not able to visit a https-website on port 8443 (Plesk control panel uses this port).
Then I allow visiting websites on port 8443 on the proxy (squid).
My internal clients are now able to visit the Plesk panel.
So my question is if proxy rules overwrite firewall rules ?!