After some further investigation this is one way to do debugging on redmine 2.3.1 under ubuntu 13.10 (ruby 1.9.3, Rails 3.2.13)

Debugging application with webrick¶

• webrick (is already installed)
• "gem install ruby-debug19" seems to be not maintained anymore see: http://stackoverflow.com/questions/1083451/debugging-in-ruby-1-9/10414984#10414984, use gem "debugger" instead (for installation instructions see the link above or short description below)
• Add the keyword "debugger" at the line in your rb file where you want to stop end debug (next, cont etc. are available)
• stop your apache webserver f.e.: "/etc/init.d/apache2 stop"
• start your webrick server in your redmine directory, connect to your redmine website (port 3000 is used in my example)

cd /usr/share/redmine
script/rails server webrick -e production -p 3000 --debug

Installation of debugger gem¶

cd /usr/share/redmine
vi Gemfile

Add one line like so:

source 'https://rubygems.org'
gem 'debugger', group: [:development, :test]

Start your server:

script/rails server webrick -e production -p 3000 --debug

In case of an error occur ...

Could not find gem 'debugger (>= 0) ruby' in the gems available on this machine.
Run `bundle install` to install missing gems.

bundle install

FWIW: for debugging on Ruby 2.x I'd recommend the use of Byebug instead.