Redmine

Your gem list shows passenger 2.2.11, your stack trace 3.0.0. I think you have a problem with the gem paths indeed, can't tell you where it is though.

Indeed the trace was showing something wrong with the gems.
Now however I have updated the $PATH and still the same even with unicorn:

root@warden:/opt/rubyEE-1.8.7-2010.02/bin# ./unicorn_rails -E production -d -c /etc/racks_apps/redmine.ru 
{:app=>
  #<Proc:0x0000000000f01108@/opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/bin/unicorn_rails:136>,
 :daemonize=>false,
 :unicorn_options=>
  {:config_file=>"/etc/racks_apps/redmine.ru", :listeners=>[]}}
Exception `Errno::EEXIST' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/fileutils.rb:243 - File exists - tmp/cache
Exception `Errno::EEXIST' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/fileutils.rb:243 - File exists - tmp/pids
Exception `Errno::EEXIST' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/fileutils.rb:243 - File exists - tmp/sessions
Exception `Errno::EEXIST' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/fileutils.rb:243 - File exists - tmp/sockets
I, [2010-10-28T12:31:17.946020 #1919]  INFO -- : listening on addr=0.0.0.0:4000 fd=3
Exception `Errno::ESRCH' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/lib/unicorn.rb:779 - No such process
I, [2010-10-28T12:31:17.949781 #1919]  INFO -- : Refreshing Gem list
Exception `NoMethodError' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/rational.rb:82 - undefined method `gcd' for Rational(1, 2):Rational
Exception `LoadError' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31 - no such file to load -- action_controller
Exception `NoMethodError' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems.rb:223 - undefined method `map' for nil:NilClass
/opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems.rb:223:in `activate': undefined method `map' for nil:NilClass (NoMethodError)
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:35:in `require'
    from /usr/share/redmine/config/environment.rb:28
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/bin/unicorn_rails:145:in `rails_builder'
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/lib/unicorn.rb:817:in `call'
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/lib/unicorn.rb:817:in `build_app!'
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/lib/unicorn.rb:269:in `start'
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/lib/unicorn.rb:29:in `run'
    from /opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/bin/unicorn_rails:210
    from ./unicorn_rails:19:in `load'
    from ./unicorn_rails:19

Gem list:

root@warden:/opt/rubyEE-1.8.7-2010.02/bin# ./gem list

*** LOCAL GEMS ***

abstract (1.0.0)
actionmailer (3.0.1, 2.3.5)
actionpack (3.0.1, 2.3.5)
activemodel (3.0.1)
activerecord (3.0.1, 2.3.5)
activeresource (3.0.1, 2.3.5)
activesupport (3.0.1, 2.3.5)
arel (1.0.1)
builder (2.1.2)
bundler (1.0.3)
daemon_controller (0.2.5)
erubis (2.6.6)
fastthread (1.0.7)
file-tail (1.0.5)
i18n (0.4.2)
mail (2.2.9)
mime-types (1.16)
mysql (2.8.1)
passenger (3.0.0)
pg (0.9.0)
polyglot (0.3.1)
rack (1.2.1, 1.0.1)
rack-mount (0.6.13)
rack-test (0.5.6)
rails (3.0.1, 2.3.5)
railties (3.0.1)
rake (0.8.7)
spruz (0.2.0)
sqlite3-ruby (1.3.1)
thor (0.14.3)
treetop (1.4.8)
tzinfo (0.3.23)
unicorn (1.1.4)

I wonder, is it possible that the ennvironment.rb is not requiring the correct version of gems if I have two versions installed?

Then, I tried to remove all 3.0.1 gems, now I get this:

root@warden:/usr/share/redmine# /opt/rubyEE-1.8.7-2010.02/bin/unicorn_rails -E production -d -c /etc/racks_apps/redmine.ru 
{:app=>
  #<Proc:0x0000000000fc7d58@/opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/bin/unicorn_rails:136>,
 :daemonize=>false,
 :unicorn_options=>
  {:config_file=>"/etc/racks_apps/redmine.ru", :listeners=>[]}}
Exception `Errno::EEXIST' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/fileutils.rb:243 - File exists - tmp/cache
Exception `Errno::EEXIST' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/fileutils.rb:243 - File exists - tmp/pids
Exception `Errno::EEXIST' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/fileutils.rb:243 - File exists - tmp/sessions
Exception `Errno::EEXIST' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/fileutils.rb:243 - File exists - tmp/sockets
I, [2010-10-28T12:58:51.289281 #2036]  INFO -- : listening on addr=0.0.0.0:4000 fd=3
Exception `Errno::ESRCH' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/unicorn-1.1.4/lib/unicorn.rb:779 - No such process
I, [2010-10-28T12:58:51.292685 #2036]  INFO -- : Refreshing Gem list
Exception `NoMethodError' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/1.8/rational.rb:82 - undefined method `gcd' for Rational(1, 2):Rational
Exception `LoadError' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31 - no such file to load -- action_controller
Exception `LoadError' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:36 - no such file to load -- action_controller
Exception `LoadError' at /opt/rubyEE-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:38 - no such file to load -- action_controller
/opt/rubyEE-1.8.7-2010.02/bin/unicorn_rails must be run inside RAILS_ROOT: #<LoadError: no such file to load -- action_controller>

Obviously:

root@warden:/usr/share/redmine# find /opt/rubyEE-1.8.7-2010.02/ -name "action_controller*" 
/opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller.rb
/opt/rubyEE-1.8.7-2010.02/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller

is there... no idea what's the cause. any advice?

You seem to be doing a lot of to-and-fro with your environment. I'd recommend trying a vanilla redmine on a vanilla MRI and all, and see if that works. If that works, you will probably need to rework your EE environment correctly.

This is a vanilla rubyEE. I have installed it twice for the tests.
It only overlaps over the default ruby in the systems that you have to have anyway.

Besides, I have two machines and I am experiencing exactly the same thing on both.
So either I am making the same mistake or Redmine doesn't want to work this way with new rubyEE...