[ruby-on-rails] Uninitialized constant ActiveSupport::Dependencies::Mutex (NameError)

I have faced this problem in many occassions when I try to start an old rails 2.3.5 project after having worked with rails 3>. In my case to solve the problem, I must do a rubygems update to version 1.4.2, this is:

sudo gem update --system 1.4.2

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