[squid-users] Cannot authenticate via LDAP. No username/password prompt

Hi everybody, I am quite new to Squid configuration, and I would like
it to authenticate our Active Directory users via LDAP. I followed the
docs and I modified the squid3.conf this way:

---
auth_param basic program /usr/lib/squid3/squid_ldap_auth  -b
"cn=Users,dc=dominio ca,dc=cc,dc=it" -f "sAMAccountName=%s"  -D
"cn=LDAP Bind User,cn=Users,dc=dominio,dc=cc,dc=it" -w
LDAPBindPassword -h server-ad
auth_param basic children 5
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 30 minutes
acl ldapauth proxy_auth REQUIRED
http_access allow ldapauth
---
When I launch squid_ldap_auth with its arguments from command line, it
works flawlessly, but when I try to access the cache I have a "Cache
Access Denied" without any username prompt.
What did I miss? Thank you in advance
-- 
Ciao
Nico