The ebuild incorrectly assumes net-proxy/squid is installed and chown therefore fails. If squid is installed after net-proxy/squidguard, then the group and user ids of /var/log/squidGuard will still be incorrect. net-proxy/squidguard needs to either: 1) create the user/group itself, or 2) depend on net-proxy/squid to be installed.
Fixed in cvs w/o revbump. I've used the first solution.