Server won't reply to POST request

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http://serverfault.com – I'm working on a WordPress site that's served by two lighttpd backends behind an F5 load balancer. Previously, the site was served by a single lighttpd server behind a nginx reverse proxy, and everything worked fine. Now, sometimes when sending POST info from the WordPress admin (for instance, when editing general options or editing navigation menus) the site won't reply anything at all. I know that it's receiving the request, because it actually updates the info, but the browser never gets a response. I'm not seeing anything unusual on the WordPress debug log or the lighttpd logs, so my gu (HowTos)