> Normally timeouts results in 504, and if you see 500 this might
> indicate that in fact request failed not due to a timeout, but
> e.g. due too loop detected. This in turn might mean that there
> were more than one request to the server X which failed.
>
> Try looking into error_log to see what's going on.

You're correct - its a 504.
[16/Nov/2012:12:40:48 +0100] "POST /url HTTP/1.1" 403 454 Time: 300.030 Upstream-time: 300.004, 0.003 Upstream: XXX, YYY Upstream-status: 504, 403