Il va falloir activer une propriété dans votre fichier squid.conf
=> ignore_expect_100 on
C'est expliqué ici : http://www.squid-cache.org/Doc/config/ignore_expect_100/
This option makes Squid ignore any Expect: 100-continue header present in the request. RFC 2616 requires that Squid being unable to satisfy the response expectation MUST return a 417 error.
Note: Enabling this is a HTTP protocol violation, but some clients may not handle it well..