Hello,
I mean that the source or destination are set to a domain name. But it seems is not your case, so forget it.
Before enabling debug mode there is other quick test you can do. Run this commands:
sudo squid -k parse /etc/squid4/squid.conf
sudo squid -k parse /etc/squid3/squid-external.conf
If they found some error, paste it there. If not, you can then try the debug mode.