Hi elliotbeken,
I have the same setup as you had. But never experience your issue.
it looks like a DNS issue, but your LAN client are connected as usual right.
I put the blocked site without the ¨www.¨
or just uninstall the HTTP Proxy (Cache and Filter) module .. and then reinstall,
do copy the squid.conf under /etc/squid/, as preference also under stubs folder, and under /etc/zentyal/hooks/squid.xxxxx if you change/add manual you own setting.
Hope it helps
Khuwera