такое ощущение, что не подхватывает твой squid авторизацию..вот пример с моего рабочего сервера (правда ещё не я настраивал), но всё работает.
icp_port 0
hierarchy_stoplist cgi-bin ?
acl QUERY urlpath_regex cgi-bin \?
no_cache deny QUERY
cache_mem 128 MB
cache_replacement_policy heap GDSF
memory_replacement_policy heap GDSF
cache_dir ufs /var/lib/squid 4000 128 1024
acl localnet src 192.168.9.0/255.255.255.0
acl homenet src 192.168.79.0/255.255.255.0
acl localhost src 127.0.0.1/255.255.255.255
acl Safe_ports port 80 443 210 70 21 1025-65535
acl CONNECT method CONNECT
acl all src 0.0.0.0/0.0.0.0
auth_param ntlm program /usr/local/samba/bin/ntlm_auth --helper-protocol=squid-2.5-ntlmssp
auth_param ntlm children 10
auth_param ntlm max_challenge_reuses 0
auth_param ntlm max_challenge_lifetime 2 minutes
auth_param basic program /usr/local/samba/bin/ntlm_auth --helper-protocol=squid-2.5-basic
auth_param basic children 10
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 2 hours
acl NTLMauth proxy_auth REQUIRED
http_access allow NTLMauth
http_access allow localnet
http_access allow homenet
http_access allow localhost
http_access deny !Safe_ports
http_access deny CONNECT
http_access deny all
cache_mgr root
cache_effective_user squid
cache_effective_group squid
logfile_rotate 0
log_icp_queries off
cachemgr_passwd my-secret-pass all
buffered_logs on