Comment 12 for bug 589611

mod_reqtimeout could be a reason, since it has been activated according to apt-listchanges:

apache2 (2.2.15-1) unstable; urgency=low

 * This release adds and enables mod_reqtimeout, which limits the time
   Apache waits for a client to send a complete request. This helps to
   mitigate against certain denial of service attacks. In case of problems
   with slow clients, the timeout values can be adjusted in
   /etc/apache2/mods-available/reqtimeout.conf , or the module can be
   disabled with "a2dismod reqtimeout".

-- Chuck Short <email address hidden> Tue, 13 Apr 2010 09:09:34 -0400

I have now disabled it and will audit the server logs whether the error has gone.