Problem occurs when requesting an url in the form 'domain.tld' (instead of 'domain.tld/').
# curl -v domain.tld
This will send a request to the (proxy) server that does not contain a trailing '/' in the path.
According to RFC 2616 the client is required to add a final '/' if the abs_path of the URI is null.
Because of this, some strict RFC 2616 http proxy server implementations will return a 'Bad Request' to Curl.