clientAccessCheck: proxy request denied in accel_only mode

Squid Cache: Version 2.5

Looks like the client is trying to use you as a proxy, either directly or indirectly via another proxy..
However, HTTP/1.1 requires HTTP servers to accept such request. Turn "httpd_accel_with_proxy on".
You should also make sure your access control rules is set correcly to only allow access to your servers if you haven't already.. Not the cause to your problem, but may be a cause to other problems later if not done..