[Rails] Initialising error on Ubuntu 11.10

After upgrading to Ubuntu 11.10 from 11.04 my Rails application does
not
start properly

$ script/server
/var/lib/gems/1.8/gems/activesupport-2.3.5/lib/active_support/
dependencies.rb:55:
uninitialized constant ActiveSupport::Dependencies::Mutex (NameError)
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in
`gem_original_require'
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in
`require'
from /var/lib/gems/1.8/gems/activesupport-2.3.5/lib/active_support.rb:
56
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in
`gem_original_require'
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in
`require'
from /var/lib/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:1
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in
`gem_original_require'
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in
`require'
from script/server:3

I am running

ruby 1.8.7 (2011-06-30 patchlevel 352) [i686-linux]

activesupport-2.3.5

Does somebody have an idea how to fix this problem?