Please note that cache_log and cache_store_log are NOT the same setting and documentation for one is not necessarily valid for the other.
'none' is apparently only valid for cache_store_log. 'cache_log none' WILL attempt to open and log to a file called 'none' as you told it to. If you *really* need to disable error reporting via cache_log you should query the squid-users mailing list for help.
