If the system is built with the -ipv6 flag, the latest stable Squid proxy is broken for HTTPS requests. This appears to be a known bug - the URL reflects a similar bug filed with Debian.
Steps to Reproduce:
1. Build system with -ipv6
2. Attempt to access an HTTPS site using Squid
The following error was encountered while trying to retrieve the URL: https://www.paypal.com/*
The system returned: (97) Address family not supported by protocol
Squid is unable to create a TCP socket, presumably due to excessive load. Please retry your request.
Your cache administrator is root.
Access to the website.
The upstream bug seems to indicate that it's fixed in 3.1.3, but that doesn't appear to be the case in Gentoo world (and perhaps not upstream either). I'm currently rebuilding my system with +ipv6 to confirm that works.
Another host system of mine is built with +ipv6 and I didn't trigger this issue, leading me to believe that is indeed the fix. I'll update in an hour or so when everything rebuilds.
Further reading into the upstream bug indicates that might be a separate issue specific to DNS handling.
Rebuilt with +ipv6 and everything works as expected. That would appear to be the workaround.
*** This bug has been marked as a duplicate of bug 331965 ***