Squid can both be used a proxy and as a cache (reverse proxy) server. In my job, it's always as the latter I meet this server.

All requests are returned with invalid HTTP header etc

The access log file looks like this:

  TCP_DENIED/400 1414 GET error:invalid-requestTCP_DENIED/400 1414 GET error:invalid-request

Solution: the acl list configuration in squid.conf listed a wrong IP address.