502 errors, upstream sent too big header

[Travis Derouin]

We seem to be getting sporadic 502 errors, mostly when users are logging in.
We run a Mediawiki based website, are running nginx 0.9.5, php-fpm 5.3.5.

The error:

2011/04/04 10:41:34 [error] 30517#0: *2143711 upstream sent too big header
while reading response header from upstream, client: 192.168.100.5, server:
_, request: "POST
/index.php?title=Special:Userlogin&action=submitlogin&type=login&returnto=Main-Page
HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "www.wikihow.com",
referrer: "http://www.wikihow.com/Special:Userlogin?returnto=Main-Page"

Our nginx.conf proxy buffer settings:

    proxy_buffers 16 16k;
    proxy_buffer_size 32k;

We've tried increasing these values from the defaults a few times, and it
doesn't seem to fix the issue.

Do you have any suggestions on how to fix this?

Thanks,
Travis
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