transparent proxy problem

[Luca Ferrari]

I'm running ubuntu server 7.04 with squid 2.6 stable, now I've got this 
strange problem. First of all I'd like to use the squid as transparent proxy, 
and thus I've got iptables redirecting all the stuff from the port 80 to the 
port 8080. If I manually set the proxy in a browser I can surf, but if I try 
to use it transparently I got a "unable to forward the request at this time" 
message error. I've found that it only happens if I take this acl enabled:

acl sbloccati_ip src "/etc/squid/sbloccati_ip.acl"
http_access allow sbloccati_ip

if I comment out the http_access line I got the squid working, and the file 
sbloccati_ip.acl contains a list of ip that are allowed to use the proxy. It 
is working with other configurations, but I cannot find the solution. I've 
got the 
httpd_port 8080 transparent
setting.
Any idea?

Thanks,
Luca