"The size of the response header is too large" for users behind proxy

Question

We have this weird issue where some clients using our CodeIgniter based web application (who are behind a proxy) are reporting that they get a "The size of the response header is too large" error when trying to use the app.

The request in question is nothing special, there's no large volumes of data being loaded, in fact it's essentially just a log-in dialog.

We are using the database to manage sessions but of course are setting some cookies etc, I have no clue where to start in debugging this (especially as I do not have access to test behind a proxy myself) so I'm looking for some guidance on common causes of such a message for users behind a proxy or advise on how I might be able to reproduce the issue myself using a package like Charles?

These are the response headers:

HTTP/1.1 200 OK
Date: Fri, 11 Apr 2014 11:00:05 GMT
Server: Apache
Set-Cookie: ihasco_user=a%3A4%3A%7Bs%3A10%3A%22session_id%22%3Bs%3A32%3A%22c2e63b3390d47d16a4ceef54d8997857%22%3Bs%3A10%3A%22ip_address%22%3Bs%3A12%3A%2282.4.172.173%22%3Bs%3A10%3A%22user_agent%22%3Bs%3A81%3A%22Mozilla%2F5.0+%28Macintosh%3B+Intel+Mac+OS+X+10.9%3B+rv%3A28.0%29+Gecko%2F20100101+Firefox%2F28.0%22%3Bs%3A13%3A%22last_activity%22%3Bi%3A1397214005%3B%7Dfa2a770bdd1fbee17d964cd059533c26; expires=Fri, 11-Apr-2014 23:00:05 GMT; path=/; domain=www.ihasco.co.uk
Set-Cookie: PHPSESSID=b74db67219e22b4f65552557f9633477; path=/
Expires: Sat, 26 Jul 1997 05:00:00 GMT
Cache-Control: private, max-age=0, must-revalidate, no-cache, no-store
Pragma: no-cache
X-UA-Compatible: IE=edge
Content-Length: 4235
Connection: close
Content-Type: text/html

TIA!

Answer 1

Ok, so after much digging it turned out that we had a redirect and before the redirect occurred a cookie was being set, however this was set inside a loop which was cycled a fairly large number of times! Head > Desk. Thanks for all the pointers though, once I gained access to the remote environment where the issue was manifesting itself it was pretty easy to see where the problem was! :)