Rails 2: Ruby gems incompatibility; downgrading gems

With gems 1.8.11 and 1.8.12, rails 2.3.5 and ruby 1.8.6
I had following problem:

rails testr2
/usr/lib/ruby/1.8/active_support/dependencies.rb:55: uninitialized constant ActiveSupport::Dependencies::Mutex (NameError)
	from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require'
	from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.8/active_support.rb:56
	from /usr/share/rails-ruby1.8/railties/bin/../lib/rails_generator.rb:28:in `require'
	from /usr/share/rails-ruby1.8/railties/bin/../lib/rails_generator.rb:28
	from /usr/share/rails-ruby1.8/railties/bin/rails:14:in `require'
	from /usr/share/rails-ruby1.8/railties/bin/rails:14

Stackoverflow described couple of solutions - adding 'require 'thread'' etc.
But it didnt work.
Finally downgrading gems helped:

sudo gem install rubygems-update -v '1.4.2'
sudo gem uninstall rubygems-update -v '1.8.12'
sudo gem uninstall rubygems-update -v '1.8.11'
sudo update_rubygems
...
gem -v
1.4.2

Will see for how long this solution lasts :)