varnish 503s when accessing backend

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http://serverfault.com – I have two backends that I'm serving on my web server, both of them are remotely hosted by third party companies. One of them is a support portal page, which is passing cookies and I suspect is causing the page to not load. I see varnish pass the request along to the backend, but when the request hits the backend I see the following response: 14 FetchError c http read error: 0 15 BackendClose b support 14 VCL_call c error Details of my VCL settings, runtime parameters and full log output can be found on my gist page: https://gist.github.com/4321241. Is there something that's not allow (HowTos)