If you are running Redmine on debian-lts (squeeze) by any chance, this issue seems to be caused by a passenger security update from the debian-lts team.
In that case you can downgrade package ibapache2-mod-passenger and hold it at the previous version:

<pre>
aptitude install libapache2-mod-passenger=2.2.11debian-2
aptitude hold libapache2-mod-passenger
</pre>

*This obviously removes the security fix to CVE-2015-7519 and you have to remember to "unhold" and update passenger as soon as this issue has been clarified/fixed.*

The update seems to cause the Rails request environent used by <pre>libactionpack-ruby1.8: /usr/lib/ruby/1.8/action_controller/request.rb</pre> to be incomplete.
In particular, <pre>@env['REQUEST_URI']</pre> seems to be <pre>nil</pre>.
(At least as far as I can tell - not a ruby expert)

Further info on the security update:
https://blog.phusion.nl/2015/12/07/cve-2015-7519/
https://lists.debian.org/debian-lts-announce/2016/01/msg00018.html