We use Squid + SquidGuard on Debian Etch for content filtering (basically URL checking) at work. It's worked for us for years, but recently we've been having intermittent trouble with Yahoo! Mail's web interface. I know our proxy server is the culprit as it works fine outside our network and it works fine when routing around the proxy server.

I've made absolutely zero changes to the server. No software updates, no configuration changes, nothing. The server appears to be in good health. For troubleshooting, I set up a second server with up-to-date software and a basic Squid configuration and the problem exists there as well!

I can provide more detail if someone out there thinks they can help. I've been poking at it for the past week or so and it's driving me bonkers!