If you’re getting errors from squid or your web browser with some weird IP address, chances are that squid is trying to connect to a server which supports IPv4 and IPv6.
To disable IPv6 on Ubuntu (also Kubuntu), add this line to the end of /etc/sysctl.conf:
net.ipv6.conf.all.disable_ipv6 = 1
Restart your computer or reload sysctl.conf by executing: sudo sysctl -p
To check, run: ip a | grep inet6
When IPv6 is disabled, this will give no output.
Source: Quick Tip: How To Disable IPv6 In Ubuntu For Better Internet Speed