RubyGem version error: rack(1.0.0 not ~> 1.0.1)

I have this error when I try to start script/server:
/opt/ruby1.8/lib/ruby/site_ruby/1.8/rubygems.rb:827:in `report_activate_error': RubyGem version error:
rack(1.0.0 not ~> 1.0.1) (Gem::LoadError)
from /opt/ruby1.8/lib/ruby/site_ruby/1.8/rubygems.rb:261:in `activate'
from /opt/ruby1.8/lib/ruby/site_ruby/1.8/rubygems.rb:68:in `gem'
from /opt/ruby1.8/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller.rb:34
from /opt/ruby1.8/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
from /opt/ruby1.8/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
from /opt/ruby1.8/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
from /opt/ruby1.8/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:521:in `new_constants_in'
from /opt/ruby1.8/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
from /opt/ruby1.8/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:2
from /opt/ruby1.8/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
from /opt/ruby1.8/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
from script/server:3

solution:
  $ gem uninstall rack
  $ gem install rack -v 1.0.1