Squid + HTTP Error 417 Solution
We've had trouble with Squid interfering with communications with different vendors (HH2, Labtech, etc.) that would produce an HTTP 417 error and had difficulty finding the appropriate resolution so I thought I would post it here.
Navigate to Services -> Proxy Server
In the "Custom Options" field add "ignore_expect_100 on" (without quotes).
Save and restart.
It's worked for us and I hope it helps someone else along the way.