You can find all ports that are allowed in the configuration file like this:

/etc/squid/squid.conf

...
acl Safe_ports port 80
...
</code>
 
Consider adding Safe_ports ACL rule for any port that your clients need.
 
----
 
You can add a port range instead of writing a rule for every port like this:
 
<file bash /etc/squid/squid.conf>
...
acl Safe_ports port 6000-7000
...

sudo systemctl restart squid