Just to help others who have issues running Squid:
If you see this in your log:
2015/07/06 11:28:43 kid1| sendto FD 12: (1) Operation not permitted
2015/07/06 11:28:43 kid1| ipcCreate: CHILD: hello write test failed
Then the solution is to allow IPv6 in System-> Settings-> Networking
thanks, it really helps.
I created a ticket and will take a look soon: https://github.com/opnsense/core/issues/271
Fixed in 15.7.6. :)